Sink/hoist memory instructions between loop fusion candidates

Currently, instructions in the preheader of the second of two fusion
candidates are sunk and hoisted whenever possible, to try to allow the
loops to fuse. Memory instructions are skipped, and are never sunk or
hoisted. This change adds memory instructions for sinking/hoisting
consideration.

This change uses DependenceAnalysis to check if a mem inst in the
preheader of FC1 depends on an instruction in FC0's header, across
which it will be hoisted, or FC1's header, across which it will be
sunk. We reject cases where the dependency is a data hazard.

Differential Revision: https://reviews.llvm.org/D131606