[MemorySSA] Teach LoopSimplify to preserve MemorySSA.
Preserve MemorySSA in LoopSimplify, in the old pass manager, if the analysis is available.
Do not preserve it in the new pass manager.
Subscribers: nemanjai, jlebar, javed.absar, Prazek, kbarton, zzheng, jsji, llvm-commits, george.burgess.iv, chandlerc
Differential Revision: https://reviews.llvm.org/D60833