[Devirtualization] Decorate vfunction load with invariant.load

Summary:
This patch was introduced one year ago, but because my google account
was disabled, I didn't get email with failing buildbot and I missed
revert of this commit. There was small but in test regex.
I am back.

Reviewers: rsmith, rengolin

Subscribers: nlewycky, rjmccall, cfe-commits

Differential Revision: https://reviews.llvm.org/D26117

llvm-svn: 285497