Don't look past volatile loads.

A volatile load should block us from trying to coalesce stores.
PR18023

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195599 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -8703,6 +8703,11 @@ bool DAGCombiner::MergeConsecutiveStores(StoreSDNode* St) {
         Index = STn;
         break;
       } else if (LoadSDNode *Ldn = dyn_cast<LoadSDNode>(NextInChain)) {
+        if (Ldn->isVolatile()) {
+          Index = NULL;
+          break;
+        }
+
         // Save the load node for later. Continue the scan.
         AliasLoadNodes.push_back(Ldn);
         NextInChain = Ldn->getChain().getNode();