This is an optimization to avoid an unneeded "mov <reg>, #0"
instruction. Instead, we can just directly use the zero register.

R=ahaas@chromium.org

Bug: chromium:854011, v8:6600
Change-Id: I187d7a659c42d7d4a6d5798eddff8b7ee0983bbc
Reviewed-on: https://chromium-review.googlesource.com/1124684
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54186}