Zir: move set_cold from Inst.Tag to Inst.Extended

If I could mark a builtin function as cold, I would mark @setCold as cold.
We have run out of `Zir.Inst.Tag`s so I had to move a tag from Zir.Inst.Tag to
Zir.Inst.Extended. This is because a new noreturn builtin will be added and
noreturn builtins cannot be part of Inst.Tag:
```
/// `noreturn` instructions may not go here; they must be part of the main `Tag` enum.
pub const Extended = enum(u16) {
```

Here's another reason I went for @setCold:
```
$ git grep setRuntimeSafety | wc -l
322
$ git grep setCold | wc -l
79
$ git grep setEvalBranchQuota | wc -l
82
```

This also simply removes @setCold from Autodoc and the docs frontend because
as far as I could understand it, builtins represented using Zir extended
instructions are not yet supported because I couldn't find
@setStackAlign or @setFloatMode there, either.

lib/docs/main.js

@@ -1187,10 +1187,6 @@ const NAV_MODES = {
             payloadHtml += "panic";
             break;
           }
-          case "set_cold": {
-            payloadHtml += "setCold";
-            break;
-          }
           case "set_runtime_safety": {
             payloadHtml += "setRuntimeSafety";
             break;

src/AstGen.zig

@@ -2609,8 +2609,9 @@ fn addEnsureResult(gz: *GenZir, maybe_unused_result: Zir.Inst.Ref, statement: As
             .extended => switch (gz.astgen.instructions.items(.data)[inst].extended.opcode) {
                 .breakpoint,
                 .fence,
-                .set_align_stack,
                 .set_float_mode,
+                .set_align_stack,
+                .set_cold,
                 => break :b true,
                 else => break :b false,
             },
@@ -2658,7 +2659,6 @@ fn addEnsureResult(gz: *GenZir, maybe_unused_result: Zir.Inst.Ref, statement: As
             .validate_struct_init_comptime,
             .validate_array_init,
             .validate_array_init_comptime,
-            .set_cold,
             .set_runtime_safety,
             .closure_capture,
             .memcpy,
@@ -8078,6 +8078,14 @@ fn builtinCall(
             });
             return rvalue(gz, ri, result, node);
         },
+        .set_cold => {
+            const order = try expr(gz, scope, ri, params[0]);
+            const result = try gz.addExtendedPayload(.set_cold, Zir.Inst.UnNode{
+                .node = gz.nodeIndexToRelative(node),
+                .operand = order,
+            });
+            return rvalue(gz, ri, result, node);
+        },
 
         .src => {
             const token_starts = tree.tokens.items(.start);
@@ -8111,7 +8119,6 @@ fn builtinCall(
         .bool_to_int           => return simpleUnOp(gz, scope, ri, node, bool_ri,                                    params[0], .bool_to_int),
         .embed_file            => return simpleUnOp(gz, scope, ri, node, .{ .rl = .{ .ty = .const_slice_u8_type } }, params[0], .embed_file),
         .error_name            => return simpleUnOp(gz, scope, ri, node, .{ .rl = .{ .ty = .anyerror_type } },       params[0], .error_name),
-        .set_cold              => return simpleUnOp(gz, scope, ri, node, bool_ri,                                    params[0], .set_cold),
         .set_runtime_safety    => return simpleUnOp(gz, scope, ri, node, bool_ri,                                    params[0], .set_runtime_safety),
         .sqrt                  => return simpleUnOp(gz, scope, ri, node, .{ .rl = .none },                           params[0], .sqrt),
         .sin                   => return simpleUnOp(gz, scope, ri, node, .{ .rl = .none },                           params[0], .sin),

src/Autodoc.zig

@@ -1338,7 +1338,6 @@ fn walkInstruction(
         .embed_file,
         .error_name,
         .panic,
-        .set_cold, // @check
         .set_runtime_safety, // @check
         .sqrt,
         .sin,

src/Sema.zig

@@ -1167,6 +1167,11 @@ fn analyzeBodyInner(
                         i += 1;
                         continue;
                     },
+                    .set_cold => {
+                        try sema.zirSetCold(block, extended);
+                        i += 1;
+                        continue;
+                    },
                     .breakpoint => {
                         if (!block.is_comptime) {
                             _ = try block.addNoOp(.breakpoint);
@@ -1304,11 +1309,6 @@ fn analyzeBodyInner(
                 i += 1;
                 continue;
             },
-            .set_cold => {
-                try sema.zirSetCold(block, inst);
-                i += 1;
-                continue;
-            },
             .set_runtime_safety => {
                 try sema.zirSetRuntimeSafety(block, inst);
                 i += 1;
@@ -5721,10 +5721,10 @@ fn zirSetAlignStack(sema: *Sema, block: *Block, extended: Zir.Inst.Extended.Inst
     gop.value_ptr.* = .{ .alignment = alignment, .src = src };
 }
 
-fn zirSetCold(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!void {
-    const inst_data = sema.code.instructions.items(.data)[inst].un_node;
-    const operand_src: LazySrcLoc = .{ .node_offset_builtin_call_arg0 = inst_data.src_node };
-    const is_cold = try sema.resolveConstBool(block, operand_src, inst_data.operand, "operand to @setCold must be comptime-known");
+fn zirSetCold(sema: *Sema, block: *Block, extended: Zir.Inst.Extended.InstData) CompileError!void {
+    const extra = sema.code.extraData(Zir.Inst.UnNode, extended.operand).data;
+    const operand_src: LazySrcLoc = .{ .node_offset_builtin_call_arg0 = extra.node };
+    const is_cold = try sema.resolveConstBool(block, operand_src, extra.operand, "operand to @setCold must be comptime-known");
     const func = sema.func orelse return; // does nothing outside a function
     func.is_cold = is_cold;
 }

src/Zir.zig

@@ -808,8 +808,6 @@ pub const Inst = struct {
         panic,
         /// Same as `panic` but forces comptime.
         panic_comptime,
-        /// Implement builtin `@setCold`. Uses `un_node`.
-        set_cold,
         /// Implement builtin `@setRuntimeSafety`. Uses `un_node`.
         set_runtime_safety,
         /// Implement builtin `@sqrt`. Uses `un_node`.
@@ -1187,7 +1185,6 @@ pub const Inst = struct {
                 .bool_to_int,
                 .embed_file,
                 .error_name,
-                .set_cold,
                 .set_runtime_safety,
                 .sqrt,
                 .sin,
@@ -1323,7 +1320,6 @@ pub const Inst = struct {
                 .validate_deref,
                 .@"export",
                 .export_value,
-                .set_cold,
                 .set_runtime_safety,
                 .memcpy,
                 .memset,
@@ -1561,7 +1557,7 @@ pub const Inst = struct {
                 => false,
 
                 .extended => switch (data.extended.opcode) {
-                    .breakpoint, .fence => true,
+                    .fence, .set_cold, .breakpoint => true,
                     else => false,
                 },
             };
@@ -1750,7 +1746,6 @@ pub const Inst = struct {
                 .error_name = .un_node,
                 .panic = .un_node,
                 .panic_comptime = .un_node,
-                .set_cold = .un_node,
                 .set_runtime_safety = .un_node,
                 .sqrt = .un_node,
                 .sin = .un_node,
@@ -1979,6 +1974,9 @@ pub const Inst = struct {
         /// Implement builtin `@setAlignStack`.
         /// `operand` is payload index to `UnNode`.
         set_align_stack,
+        /// Implements `@setCold`.
+        /// `operand` is payload index to `UnNode`.
+        set_cold,
         /// Implements the `@errSetCast` builtin.
         /// `operand` is payload index to `BinNode`. `lhs` is dest type, `rhs` is operand.
         err_set_cast,

src/print_zir.zig

@@ -196,7 +196,6 @@ const Writer = struct {
             .error_name,
             .panic,
             .panic_comptime,
-            .set_cold,
             .set_runtime_safety,
             .sqrt,
             .sin,
@@ -503,6 +502,7 @@ const Writer = struct {
             .fence,
             .set_float_mode,
             .set_align_stack,
+            .set_cold,
             .wasm_memory_size,
             .error_to_int,
             .int_to_error,