emitIAS for movsx and movzx.

Force dest to be the full 32-bit reg instead of sometimes being
a 16-bit reg. This is to save on a operand size prefix (and
avoid passing the DestTy down to the dispatchers).

BUG=none
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/647223004
7 files changed