Use MCRegister in MCRegisterInfo's interfaces

Summary:
As part of this, define DenseMapInfo for MCRegister (and Register while I'm at it)

Depends on D65599

Reviewers: arsenm

Subscribers: MatzeB, qcolombet, jvesely, wdng, nhaehnle, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65605

llvm-svn: 367719