The fast path of all write barriers already got mostly unified in
https://crrev.com/c/3644964. However, the shared heap write barrier
still added a new branch in the fast path of the full write barrier.

This CL unifies the branch for the generational and the shared heap
write barrier in the fast path at the cost of an additional branch in
the slow path. This should hopefully the rest of the regressions caused
by introducing the shared heap write barrier.

Bug: chromium:1326446, v8:11708
Change-Id: Id5a8334c50a7455e53caf65891d4304d9d2e7702
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3663091
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80749}