[EarlyCSEwMemorySSA] Add MSSA verification and tests to make EarlyCSE failures easier to track.

Summary:
EarlyCSE can make IR changes that will leave MemorySSA with accesses claiming to be optimized, but for which a subsequent MemorySSA run will yield a different optimized result.
Due to relying on AA queries, we can't fix this in general, unless we recompute MemorySSA.
Adding some tests to track this and a basic verify for future potential failures.

Reviewers: george.burgess.iv, gberry

Subscribers: sanjoy, jlebar, Prazek, llvm-commits

Differential Revision: https://reviews.llvm.org/D51960

llvm-svn: 342422