Also, their priority is lowered so that registers which are scratch on both
Unix and Windows are preferred by the register allocator.

BUG=swiftshader:22

Change-Id: Id55d8c8b8c106947e3041a082099069d7c6c6ed0
Reviewed-on: https://chromium-review.googlesource.com/404503Reviewed-by: Jim Stichnoth <stichnot@chromium.org>
Tested-by: Jim Stichnoth <stichnot@chromium.org>