[LoopSimplifyCFG] Update MemorySSA after r353911.

Summary:
MemorySSA is not properly updated in LoopSimplifyCFG after recent changes. Use SplitBlock utility to resolve that and clear all updates once handleDeadExits is finished.
All updates that follow are removal of edges which are safe to handle via the removeEdge() API.
Also, deleting dead blocks is done correctly as is, i.e. delete from MemorySSA before updating the CFG and DT.

Reviewers: mkazantsev, rtereshin

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58524

llvm-svn: 354613