Add a test to check that the encodings are efficient for immediates
(chooses the i8, and eax encodings when appropriate).

The .byte syntax breaks NaCl bundle straddle checking in llvm-mc,
so I had to change one of the tests which noted that a nop appeared
(no longer does).

This also assumes that _add(), etc. are usually done with _add(T, ...) and
then _mov(dst, T) so that the dest is always register.

BUG=none
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/604873003