Expose must/may alias info in MemorySSA.
Building MemorySSA gathers alias information for Defs/Uses.
Store and expose this information when optimizing uses (when building MemorySSA),
and when optimizing defs or updating uses (getClobberingMemoryAccess).
Current patch does not propagate alias information through MemoryPhis.
Reviewers: gbiv, dberlin
Subscribers: Prazek, sanjoy, llvm-commits
Differential Revision: https://reviews.llvm.org/D38569