[LoopSimplifyCFG] Update MemorySSA in terminator folding. PR39783

Terminator folding transform lacks MemorySSA update for memory Phis,
while they exist within MemorySSA analysis. They need exactly the same
type of updates as regular Phis. Failing to update them properly ends up
with inconsistent MemorySSA and manifests in various assertion failures.

This patch adds Memory Phi updates to this transform.

Thanks to @jonpa for finding this!

Differential Revision: https://reviews.llvm.org/D55050
Reviewed By: asbirlea

llvm-svn: 347979