The SWEEPING phase in incremental marking was used to finish sweeping
of the last GC cycle concurrently before starting incremental marking.
This avoids potentially long pauses when starting incremental marking.
However this shouldn't be necessary in most cases where sweeping is
already finished when starting the next cycle. The implementation also
didn't cleanly separate the GC cycles.

In case the sweeping phase is necessary for pause times, we can
introduce a "CompleteSweep" phase which runs right before starting
incremental marking.

Change-Id: Iaff8c06d5691e584894f57941f181d0424051eec
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2567707
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71555}