[Devirtualization] Emit invariant.group loads with empty group md

Summary:
As discussed here
http://lists.llvm.org/pipermail/llvm-dev/2017-January/109332.html
having different groups doesn't solve the problem entirly.

Reviewers: rjmccall, rsmith

Subscribers: amharc, cfe-commits

Differential Revision: https://reviews.llvm.org/D32110

llvm-svn: 301178