[GuardWidening] Preserve MemorySSA

As reported on https://bugs.llvm.org/show_bug.cgi?id=51020, the
guard widening pass doesn't preserve MemorySSA, so it can no
longer be scheduled in the same loop pass manager as LICM. However,
the loop-schedule.ll test indicates that this is supposed to work.

Fix this by preserving MemorySSA if available, as this seems to be
trivial in this case (we only need to drop the memory access for
the removed guards).

Differential Revision: https://reviews.llvm.org/D108386