[MachineOperand] Add a ChangeToGA method

Summary:
Analogous to the other ChangeToXXX methods. See the next patch for a
use case.

Change-Id: I6548d614706834fb9109ab3c8fe915e9c6ece2a7

Reviewers: arsenm, kzhuravl

Subscribers: wdng, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61651

llvm-svn: 360789