Thought leaving "mov" simple and not handle memory operands,
but then we'd have to duplicate some of the lowerAssign code
for lowerLoad =/

BUG=  https://code.google.com/p/nativeclient/issues/detail?id=4076
R=kschimpf@google.com, stichnot@chromium.org

Review URL: https://codereview.chromium.org/1152703006