This CL contains minor refactorings to some parts of the garbage
collector:

- Space iterators.
- Removes a redundant call to Heap::CreateFillerObjectAt.
- Heap::CompleteSweepingFull now ensures that sweeping in the C++
  managed heap is also completed.
- Checks, comments and code cleanup.

Change-Id: I14a7fe45c270c463c94c86f45b0e65757249d548
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3377125Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Nikolaos Papaspyrou <nikolaos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78581}