Move register namespacing definitions from TargetRegisterInfo to Register

Summary:
The namespacing in Register is currently slightly wrong as there is a
(rarely used) stack slot namespace too. The namespacing doesn't use
anything from the Target so we can move the definition from
TargetRegisterInfo to Register to keep it in one place

Note: To keep the patch reasonably sized for review I've left stub
functions in the original TargetRegisterInfo. We should update all the uses
instead

Reviewers: arsenm, bogner, aditya_nandakumar, volkan

Subscribers: wdng, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65553

llvm-svn: 367614