Expose must/may alias info in MemorySSA.

Summary:
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

llvm-svn: 327035