[MergedLoadStoreMotion] Merge stores with conflicting value types

Since memory does not have an intrinsic type, we do not need to require value type matching on stores in order to sink them. To facilitate that, this patch finds stores which are sinkable, but have conflicting types, and bitcasts the ValueOperand so they are easily sinkable into a PHINode. Rather than doing fancy analysis to optimally insert the bitcast, we always insert right before the relevant store in the diamond branch. The assumption is that later passes (e.g. GVN, SimplifyCFG) will clean up bitcasts as needed.

Differential Revision: https://reviews.llvm.org/D147348