During sweeeping/compaction the bitmap is being reconstructed and
should not be relied on for finding object start.
Add a DCHECK that the bitmap is fully populated.

Bug: chromium:1307471
Change-Id: I4aa414722262bb6fb169123a49fce1510a60d3ef
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3540680Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79575}