_Don't run mergestore if store combining is on

Store combining is a super set of mergestore. If store combining
is on, do not run mergestore as it is not needed.

Don't expect functional change.

Author: jgu222

IGC/Compiler/CISACodeGen/MemOpt.cpp

@@ -520,6 +520,9 @@ bool MemOpt::runOnFunction(Function& F) {
     if (ProfitVectorLengths.empty())
         buildProfitVectorLengths(F);
 
+    // If LdStCombining is on, no need to do memopt.
+    const bool LdStCombineOn = doLdStCombine(CGC);
+
     bool Changed = false;
 
     IGC::IGCMD::FunctionInfoMetaDataHandle funcInfoMD = MDU->getFunctionsInfoItem(&F);
@@ -568,8 +571,10 @@ bool MemOpt::runOnFunction(Function& F) {
 
             if (LoadInst * LI = dyn_cast<LoadInst>(I))
                 Changed |= mergeLoad(LI, MI, MemRefs, MemRefsToOptimize);
-            else if (StoreInst * SI = dyn_cast<StoreInst>(I))
-                Changed |= mergeStore(SI, MI, MemRefs, MemRefsToOptimize);
+            else if (StoreInst* SI = dyn_cast<StoreInst>(I)) {
+                if (!LdStCombineOn)
+                    Changed |= mergeStore(SI, MI, MemRefs, MemRefsToOptimize);
+            }
             else if (EnableRemoveRedBlockreads) {
                 if (GenIntrinsicInst* GInst = dyn_cast<GenIntrinsicInst>(I)) {
                     if (GInst->getIntrinsicID() == GenISAIntrinsic::GenISA_simdBlockRead) {