There are many occurrences of if (RegNum == RegNumT::NoRegister).

This patch eliminates NoRegister and provides a simpler mechanism for declaring and testing RegNumT values to see if they are undefined.

BUG= none
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1691193002 .

Patch from Reed Kotler <rkotlerimgtec@gmail.com>.