Add MCRegister and use it in MCRegisterClass::contains()

Summary:
Register can cast to MCRegister and we may want to consider asserting
!isValid() || isPhysical() when expensive checks are on.

Depends on D65554

Reviewers: arsenm

Subscribers: wdng, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65599

llvm-svn: 367711