r8, r12, and r15 should be usable as general allocatable registers.
This is a performance experiment. If it causes more regressions than
improvements, we can simply revert it.

Change-Id: I757c06e9d0fc760e900b228b92671d6710bf4560
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3782672
Auto-Submit: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81897}