x86_64: fix packed struct equality

Closes #22990

test/behavior/packed-struct.zig

@@ -1307,6 +1307,17 @@ test "packed struct equality" {
     comptime try S.doTest(x, y);
 }
 
+test "packed struct equality ignores padding bits" {
+    if (builtin.zig_backend == .stage2_c) return error.SkipZigTest;
+    if (builtin.zig_backend == .stage2_spirv64) return error.SkipZigTest;
+
+    const S = packed struct { b: bool };
+    var s: S = undefined;
+    s.b = true;
+    try std.testing.expect(s != S{ .b = false });
+    try std.testing.expect(s == S{ .b = true });
+}
+
 test "packed struct with signed field" {
     var s: packed struct {
         a: i2,