[LICM & MemorySSA] Don't sink/hoist stores in the presence of ordered loads.
Before this patch, if any Use existed in the loop, with a defining
access in the loop, we conservatively decide to not move the store.
What this approach was missing, is that ordered loads are not Uses, they're Defs
in MemorySSA. So, even when the clobbering walker does not find that
volatile load to interfere, we still cannot hoist a store past a
Subscribers: sanjoy, jlebar, Prazek, llvm-commits
Differential Revision: https://reviews.llvm.org/D59564