This CL removes String.fromCharCode from comparisons and uses
a WordEqual on char codes if possible.

Bug: v8:7531
Change-Id: Idb3529d4709df3976bf92f3ddb51f81de54c7465
Reviewed-on: https://chromium-review.googlesource.com/960082
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51905}