Subzero. X86. Refactors initRegisterSet.

initRegisterSet() for x8632 and x8664 were both huge. This CL refactors
those methods to use a pre-initialized table instead of the result of
expanding the x-macros.

BUG=
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1546373003 .
5 files changed