The testb instruction requires the REX prefix when either of its
operands uses a register with the high bit set. The existing code only
considered the register operand. In the test case the REX prefix was not
emitted because the testb instruction had the register operand RAX which
does not have the high bit set. The REX prefix was necessary though
because the memory operand used R8, which has the high bit set.

R=bmeurer@chromium.org
BUG=chromium:688876

Change-Id: Ib214bebbe75965664f2aea530e29afa95a54f44f
Reviewed-on: https://chromium-review.googlesource.com/439145
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43030}