[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

Change-Id: Id65712d1f3904780877403e554ac065be03d6db0
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@346654 91177308-0d34-0410-b5e6-96231b3b80d8