This CL refactors the implementation of inner pointer resolution, based
on the marking bitmap. MarkCompactCollector::FindBasePtrForMarking has
most of its code that processes the marking bitmap moved to a utility
function FindPreviousObjectForConservativeMarking, which iterates
backwards to find the closest previous object on the page that has been
marked.

Bug: v8:12851
Change-Id: I980ac5712d8b1df792196d77edb9526ca2e13e2c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3758227Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Nikolaos Papaspyrou <nikolaos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81750}