Update MemorySSA in LoopSimplifyCFG.

Summary:
Add MemorySSA as a dependency to LoopSimplifyCFG and preserve it.
Disabled by default until all passes preserve MemorySSA.

Reviewers: bogner, chandlerc

Subscribers: sanjoy, jlebar, Prazek, george.burgess.iv, llvm-commits

Differential Revision: https://reviews.llvm.org/D50911

llvm-svn: 340445