Specifically, we were ending up with Encoded_Reg_xmm0=0 yet Encoded_Reg_xmm1=10, Encoded_Reg_xmm2=11, etc.

It's a mystery as to why this wasn't triggering any failures with filetype!=asm.

BUG= none
R=jpp@chromium.org

Review URL: https://codereview.chromium.org/1231973003.