Previously, StoreStoreElimination handled allocations as
"can observe anything". This is pretty conservative and prohibits
elimination of repeated double stores to the same field.
With this CL allocations are changed to "observes initializing stores".
This way it is guaranteed that initializing stores to a freshly created
object are not eliminated before allocations (that can trigger GC), but
allows elimination of non-initializing, unobservable stores in the
presence of allocations.

Bug: v8:12200
Change-Id: I5ef1ca8892a84a3b332e081e2fa6285d0eba9d46
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3211585
Commit-Queue: Patrick Thier <pthier@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77299}