At the end of scavenge, the GC iterates all pages to search for empty
buckets and free them. With this CL the scavenger marks buckets
(and their corresponding pages) that were empty to reduce work.
After finishing scavenging the GC only needs to revisit those marked
buckets.

Unlike (minor) mark-compact, the scavenger can't directly free those
buckets, since the evacuation and pointer updating-phases are
not separated.

Right now the pages are processed sequentially but this could be
parallelized in a subsequent CL.

Change-Id: I47ed8c0e952b06c5d960e39a6f38e745d5618656
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1889884
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64781}