Refactor code such that the linear algorithm is actually executed
outside the method for the fixpoint iteration. Also added a CHECK
which verifies that iterating the ephemerons one more time results in
no further marked objects.

Also force another iteration when ProcessMarkingWorklist() processed
some object. In such cases we need to re-process all ephemerons
otherwise ephemeron semantics might be broken.

Bug: chromium:1252918
Change-Id: I9123dda50e34227a04825fd8b3172368286cc76f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3190100
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77131}