PopToModifiableRegister did not check the {pinned} list, so it could
return a register which was already used for another (temporary) value.
This CL fixes that, and adds a little optimization which gives more
freedom to the choice of spilling and has a chance to avoid a register
mode.

R=jkummerow@chromium.org

Bug: chromium:1337221
Change-Id: Ifc02321038713ff03e8f8e7db78dde33f70ec847
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3707287Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81255}