Assert that we never copy properties from an object itself

When copying properties, it should never happen that source == target.
Add a CHECK to assert this assumption.

Bug: chromium:1260129
Change-Id: Ia5248e4363d85e13052db726fb7143897cea9c87
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3226779
Commit-Queue: Patrick Thier <pthier@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77418}

src/objects/js-objects.cc

@@ -230,6 +230,9 @@ V8_WARN_UNUSED_RESULT Maybe<bool> FastAssign(
     return Just(false);
   }
 
+  // We should never try to copy properties from an object itself.
+  CHECK_IMPLIES(!use_set, !target.is_identical_to(from));
+
   Handle<DescriptorArray> descriptors(map->instance_descriptors(isolate),
                                       isolate);