ppc: [liftoff] optimize FillStackSlotsWithZero

Change-Id: Ic2576da8adff6935758ecae14ce5441d8af6428b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3075123


Reviewed-by: Milad Fa <mfarazma@redhat.com>
Commit-Queue: Junliang Yan <junyan@redhat.com>
Cr-Commit-Position: refs/heads/master@{#76121}