Revert Register/MCRegister: Add conversion operators to avoid use of implicit...

Revert Register/MCRegister: Add conversion operators to avoid use of implicit convert to unsigned. NFC

MSVC finds ambiguity where clang doesn't and it looks like it's not going to be an easy fix
Reverting while I figure out how to fix it

This reverts r367916 (git commit aa15ec3c)
This reverts r367920 (git commit 5d14efe2)

llvm-svn: 367932