We used to scan the full heap of all clients for pointers into the
shared heap. Now that the OLD_TO_SHARED remembered set is considered
complete, we don't need to scan objects in the old generation anymore.
Instead we just need to check slots in the OLD_TO_SHARED remembered
set.

Since we don't have write barriers for objects guaranteed to be in
new space, we still need to scan objects in the young generation.

Bug: v8:11708
Change-Id: I1121f90ee63521c9141431f4cb31973796f1a67c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3865561Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82941}