Only remove old-to-old slots during a GC, but DCHECK that the
old-to-old-slot set is empty after a full GC.

Previously we simply removed from the remembered set during and outside
the full GC. We now have a flag to DCHECK this more precisely.

Bug: v8:12760
Change-Id: Ie6adc3f47a700497aaa818da0e83d6cb94e3c75d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3562981Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79798}