[MemorySSA] Invalidate MemorySSA if AA or DT are invalidated.

Summary:
MemorySSA keeps internal pointers of AA and DT.
If these get invalidated, so should MemorySSA.

Reviewers: george.burgess.iv, chandlerc

Subscribers: jlebar, Prazek, llvm-commits

Tags: LLVM

Differential Revision: https://reviews.llvm.org/D61043

llvm-svn: 359627