[DAGCombiner] Fix load-store forwarding of indexed loads.

Summary:
Handle extra output from index loads in cases where we wish to
forward a load value directly from a preceeding store.

Fixes PR39571.

Reviewers: peter.smith, rengolin

Subscribers: javed.absar, hiraditya, arphaman, llvm-commits

Differential Revision: https://reviews.llvm.org/D54265

llvm-svn: 346654