The InstructionSelector on x64 was missing the ability to properly match
comparisons of memory operands with zero, i.e. it used to turn something
like

  Word32Equal(Load[Uint8](o, i), Int32Constant(0))

into

  movzbl reg, [o,i]
  cmp 0, reg

even requiring a temporary register. Now with this change it generates
the proper

  cmpb [o,i], 0

sequence.

R=sigurds@chromium.org

Bug: v8:8238
Change-Id: I52a71bbf95c85e11cb275f0f4a5726a6873cde95
Reviewed-on: https://chromium-review.googlesource.com/c/1281342Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56677}