[x64] Use xorl to materialize smi zero.
[x64] Use xorl to materialize smi zero.

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

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

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#27829}

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh