[NFCI] Cleanup range checks in Register/MCRegister

Summary:
by removing casts from unsigned to int that which may be implementation
defined according to C++14 (and thus trip up the XL compiler on AIX) by
just using unsigned comparisons/masks and refactor out the range
constants to cleanup things a bit while we are at it.

Reviewers: hubert.reinterpretcast, arsenm

Reviewed By: hubert.reinterpretcast

Subscribers: wdng, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82197