Fix a cycle in the DAG. In this code we replace multiple loads with a single load and

multiple stores with a single load. We create the wide loads and stores (and their chains)
before we remove the scalar loads and stores and fix the DAG chain. We attempted to merge
loads with a different chain. When that happened, the assumption that it is safe to RAUW
broke and a cycle was introduced.

llvm-svn: 165148