Split OLD_TO_NEW remembered set and add OLD_TO_NEW_SWEEPING. The
OLD_TO_NEW remembered set is moved to OLD_TO_NEW_SWEEPING during
mark-compact. OLD_TO_NEW_SWEEPING is then modified by the sweeper.
Before using the page again, OLD_TO_NEW and OLD_TO_NEW_SWEEPING are
merged again.

This means only the main thread modifies OLD_TO_NEW, the sweeper only
removes entries from OLD_TO_NEW_SWEEPING. We can use this property
to make accesses non-atomic in a subsequent CL.

Bug: v8:9454
Change-Id: I9057cf85818d647775ae4c7beec4c8ccf73e18f7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1771783Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63807}