[DAGCombiner] Add an option to control whether or not to enable store merging

[wmi]

Summary

Add an option to control whether or not to enable store merging in dag combiner so we can workaround some bug more easily.

[tejohnson]

lgtm

llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

 StressLoadSlicing("combiner-stress-load-slicing", cl::Hidden,
                   cl::desc("Bypass the profitability model of load slicing"),
                   cl::init(false));
 
 static cl::opt<bool>
   MaySplitLoadIndex("combiner-split-load-index", cl::Hidden, cl::init(true),
                     cl::desc("DAG combiner may split indexing from loads"));
 
+static cl::opt<bool>
+    EnableStoreMerging("combiner-store-merging", cl::Hidden, cl::init(true),
+                       cl::desc("DAG combiner enable merging multiple stores "
+                                "into a wider store"));
+
 static cl::opt<unsigned> TokenFactorInlineLimit(
     "combiner-tokenfactor-inline-limit", cl::Hidden, cl::init(2048),
     cl::desc("Limit the number of operands to inline for Token Factors"));
 
 namespace {
 
   class DAGCombiner {
     SelectionDAG &DAG;
bool DAGCombiner::checkMergeStoreCandidatesForDependencies(
   for (unsigned i = 0; i < NumStores; ++i)
     if (SDNode::hasPredecessorHelper(StoreNodes[i].MemNode, Visited, Worklist,
                                      Max))
       return false;
   return true;
 }
 
 bool DAGCombiner::MergeConsecutiveStores(StoreSDNode *St) {
-  if (OptLevel == CodeGenOpt::None)
+  if (OptLevel == CodeGenOpt::None || !EnableStoreMerging)
     return false;
 
   EVT MemVT = St->getMemoryVT();
   int64_t ElementSizeBytes = MemVT.getStoreSize();
   unsigned NumMemElts = MemVT.isVector() ? MemVT.getVectorNumElements() : 1;
 
   if (MemVT.getSizeInBits() * 2 > MaximumLegalStoreInBits)
     return false;