This removes a redundant "mov <reg>, #0" by spilling the zero register
directly.

Change-Id: I608f5200ee25e176f6d3279aec07c581d9f7d769
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1517872Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Martyn Capewell <martyn.capewell@arm.com>
Cr-Commit-Position: refs/heads/master@{#60192}