[BranchFolding] do not iterate the aliases of virtual registers

Summary:
MCRegAliasIterator only works for physical registers. So, do not run it
on virtual registers.

With this issue fixed, we can resurrect the BranchFolding pass in NVPTX
backend.

Reviewers: jholewinski, bkramer

Subscribers: henryhu, meheff, llvm-commits, jholewinski

Differential Revision: http://reviews.llvm.org/D11174

llvm-svn: 242871