The optimization was replacing
  String.fromCharCode(x) == "y"
with x == y instead of (x & 0xFFFF) == y if x was outside
of uint16 range.

Bug: v8:7340, v8:7531
Change-Id: I967306cc2e05c28de82e16cf1b2312fe47396a7d
Reviewed-on: https://chromium-review.googlesource.com/979808Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52214}