Refactors code to take advantage of these instructions with
Uxtb/Uxth.

Note. These instructions are used by Subzero, but not Dart.

BUG= https://bugs.chromium.org/p/nativeclient/issues/detail?id=4334
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1503273002 .