AstGen: cleanup previous fix

Allocating an extended tag is much cleaner and easier to reason about
than reusing an existing tag.  The previous `.data = undefined` was a
clear indication that we don't have any data to store, and so might as
well store an extended tag in that space almost for free.

src/AstGen.zig

@@ -2906,9 +2906,14 @@ fn deferStmt(
         try gz.addDbgBlockBegin();
         const ident_name = try gz.astgen.identAsString(payload_token);
         remapped_err_code = @intCast(Zir.Inst.Index, gz.astgen.instructions.len);
-        // Use a placeholder tag of .as to allow querying things that depend on the tag,
-        // but undefined data to prevent querying of data.bin.
-        try gz.astgen.instructions.append(gz.astgen.gpa, .{ .tag = .as, .data = undefined });
+        try gz.astgen.instructions.append(gz.astgen.gpa, .{
+            .tag = .extended,
+            .data = .{ .extended = .{
+                .opcode = .errdefer_err_code,
+                .small = undefined,
+                .operand = undefined,
+            } },
+        });
         const remapped_err_code_ref = Zir.indexToRef(remapped_err_code);
         local_val_scope = .{
             .parent = &defer_gen.base,

src/Sema.zig

@@ -1198,6 +1198,7 @@ fn analyzeBodyInner(
                         i += 1;
                         continue;
                     },
+                    .errdefer_err_code => unreachable, // never appears in a body
                 };
             },
 

src/Zir.zig

@@ -1979,7 +1979,7 @@ pub const Inst = struct {
         /// `operand` is `src_node: i32`.
         breakpoint,
         /// Implements the `@select` builtin.
-        /// operand` is payload index to `Select`.
+        /// `operand` is payload index to `Select`.
         select,
         /// Implement builtin `@errToInt`.
         /// `operand` is payload index to `UnNode`.
@@ -1999,7 +1999,7 @@ pub const Inst = struct {
         /// `operand` is payload index to `Cmpxchg`.
         cmpxchg,
         /// Implement the builtin `@addrSpaceCast`
-        /// `Operand` is payload index to `BinNode`. `lhs` is dest type, `rhs` is operand.
+        /// `operand` is payload index to `BinNode`. `lhs` is dest type, `rhs` is operand.
         addrspace_cast,
         /// Implement builtin `@cVaArg`.
         /// `operand` is payload index to `BinNode`.
@@ -2031,6 +2031,9 @@ pub const Inst = struct {
         /// Implements the `@inComptime` builtin.
         /// `operand` is `src_node: i32`.
         in_comptime,
+        /// Used as a placeholder for the capture of an `errdefer`.
+        /// This is replaced by Sema with the captured value.
+        errdefer_err_code,
 
         pub const InstData = struct {
             opcode: Extended,

src/print_zir.zig

@@ -467,6 +467,7 @@ const Writer = struct {
             .breakpoint,
             .c_va_start,
             .in_comptime,
+            .errdefer_err_code,
             => try self.writeExtNode(stream, extended),
 
             .builtin_src => {