Consolidate MemRefs handling from BranchFolding and correct latent bug

Consolidate MemRefs handling from BranchFolding and correct latent bug

Move the logic from BranchFolding to use the shared infrastructure for merging MMOs introduced in 256909. This has the effect of making BranchFolding more capable.

In the process, fix a latent bug. The existing handling for merging didn't handle the case where one of the instructions being merged had overflowed and dropped MemRefs. This was a latent bug in the places the code was commoned from, but potentially reachable in BranchFolding.

Once this is in, we're left with a single place to consider implementing MMO unique-ing as proposed in http://reviews.llvm.org/D15230.

Differential Revision: http://reviews.llvm.org/D15913

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@256966 91177308-0d34-0410-b5e6-96231b3b80d8