AstGen: add missing rvalue call to labeledBlockExpr

...and fix a minor x86_64 backend bug exposed by this fix.

Resolves: #21974

lib/std/zig/AstGen.zig

@@ -2435,6 +2435,7 @@ fn blockExpr(
         if (!block_scope.endsWithNoReturn()) {
             // As our last action before the break, "pop" the error trace if needed
             _ = try gz.addRestoreErrRetIndex(.{ .block = block_inst }, .always, block_node);
+            // No `rvalue` call here, as the block result is always `void`, so we do that below.
             _ = try block_scope.addBreak(.@"break", block_inst, .void_value);
         }
 
@@ -2531,7 +2532,8 @@ fn labeledBlockExpr(
     if (!block_scope.endsWithNoReturn()) {
         // As our last action before the return, "pop" the error trace if needed
         _ = try gz.addRestoreErrRetIndex(.{ .block = block_inst }, .always, block_node);
-        _ = try block_scope.addBreak(.@"break", block_inst, .void_value);
+        const result = try rvalue(gz, block_scope.break_result_info, .void_value, block_node);
+        _ = try block_scope.addBreak(.@"break", block_inst, result);
     }
 
     if (!block_scope.label.?.used) {

src/arch/x86_64/Lower.zig

@@ -532,6 +532,8 @@ fn emit(lower: *Lower, prefix: Prefix, mnemonic: Mnemonic, ops: []const Operand)
                             },
                             else => unreachable,
                         };
+                    } else {
+                        return lower.fail("TODO: bin format '{s}'", .{@tagName(lower.bin_file.tag)});
                     }
                 },
             },

test/cases/compile_errors/labeled_block_implicit_value.zig

@@ -0,0 +1,10 @@
+export fn foo() void {
+    const result: u32 = b: {
+        if (false) break :b 1;
+    };
+    _ = result;
+}
+
+// error
+//
+// :2:28: error: expected type 'u32', found 'void'