[MemDep] Fixed handling of invariant.group

Summary:
Memdep had funny bug related to invariant.groups - because it did not
invalidated cache, in some very rare cases it was possible to show memory
dependence of the instruction that was deleted, but because other
instruction took it's place it resulted in call to vtable!
Thanks @amharc for repro!.

Reviewers: dberlin, kuhar, amharc

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D45320



Co-authored-by: Krzysztof Pszeniczny <krzysztof.pszeniczny@gmail.com>
llvm-svn: 332781