Before we always loaded smi zero via a movabs with a 64-bit immediate,
which is pretty expensive compared to the xorl.

R=jarin@chromium.org

Committed: https://crrev.com/f236777bfe6e080ff1ead6baf847cc9b6bb4f9cb
Cr-Commit-Position: refs/heads/master@{#27829}

Review URL: https://codereview.chromium.org/1085153002

Cr-Commit-Position: refs/heads/master@{#32784}