The checks for assignemnts to member during prefinalizers assumed the
slot has to live. It was assumed that if a slot is dead then we would
not be updating it.
Prefinalizers are allowed to touch dead objects and thus are techincally
allowed to write to dead slots. Such writes are usually redundant (the
object will be swept soon anyway) but are not always easy to get rid of.

Bug: chromium:1255152, v8:11749
Change-Id: I57e143abd53d434c3198616909c506eb70d8944b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3199800Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77208}