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

Summary:
This has no functional effect but makes it more obvious which parts of the
compiler do not use Register/MCRegister when you mark the implicit conversion
deprecated.

Implicit conversions for comparisons accounted for ~20% (~3k of ~13k) of
the implicit conversions when I first measured it. I haven't maintained
those numbers as other patches have landed though so it may be out of date.

Reviewers: arsenm

Reviewed By: arsenm

Subscribers: wdng, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65678

llvm-svn: 367916