Prevent vregs leaking into the MC layer via TargetRegisterClass::contains()

Summary:
The MC layer doesn't expect to deal with vregs but
TargetRegisterClass::contains() forwards into MCRegisterClass::contains()
and this can cause vregs to turn up in the MC layer APIs. Add guards
against this to prevent this becoming a problem as we replace unsigned
with a new MCRegister object for improved type safety.

Reviewers: arsenm

Subscribers: wdng, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65554

llvm-svn: 367636