[DSE] Support traversing MemoryPhis.

For MemoryPhis, we have to avoid that the MemoryPhi may be executed
before before the access we are currently looking at.

To do this we do a post-order numbering of the basic blocks in the
function and bail out once we reach a MemoryPhi with a larger (or equal)
post-order block number than the current MemoryAccess.
This changes the order in which we visit stores for elimination.

This patch also adds support for exploring multiple paths. We keep a worklist (ToCheck) of memory accesses that might be eliminated by our starting MemoryDef or MemoryPhis for further exploration.  For MemoryPhis, we add the incoming values to the worklist, for MemoryDefs we add the defining access.

Reviewers: dmgreen, rnk, efriedma, bryant, asbirlea

Reviewed By: asbirlea

Differential Revision: https://reviews.llvm.org/D72148