Extend the bswap test to have a case which will exhibit a bit of register
pressure to test register encoding more (at first wasn't sure if it was
0xC8 + reg or 0xC8 | reg... but it should be the same since there's only
0-7 for regs).

BUG=none
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/595093002