When updating pointers during a full GC, a page might not be swept
already. In such cases there might be invalid objects and slots recorded
in free memory. Updating tagged slots in free memory is fine even though
it is superfluous work.

However, the GC also needs to calculate the size of potentially dead
invalid objects in order to be able to check whether a slot is within
that object. But since that object is dead, its map might be dead as
well which makes size calculation impossible on such objects. The CL
changes this to cache the size of invalid objects. A follow-up CL will
also check the marking bit of invalid objects.

Bug: v8:12578, chromium:1316289
Change-Id: Ie773d0862a565982957e0dc409630d76552d1a32
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3599482Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Jakob Linke <jgruber@chromium.org>
Reviewed-by: Patrick Thier <pthier@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80169}