So far all OLD_TO_SHARED slots were deleted after a shared GC. The
remembered set was rebuilt in the next shared GC from scratch. This CL
changes this behavior to only remove slots that don't point into the
shared heap anymore.

We still need to remove the full OLD_TO_SHARED slot set for young
generation pages though. During a shared GC we use the OLD_TO_SHARED
remembered set to cache references into the shared heap even for
pages in the young generation to avoid the second new space object
iteration.

Bug: v8:11708
Change-Id: If92fca25e8fe7e7bf5fc5562c974b0d4c121cb02
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3790967
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82078}