This CL re-structures the write barrier slow path in order to prepare
for adding the shared write barrier. Behavior remains the same in this
CL, only code structure changes a bit (e.g. the branch for when
marking is off, got moved up to the IsMarking() check).

Bug: v8:13018
Change-Id: I991f896abb88e0c85de3123fa67d8f47282f632d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3771840
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81836}