[DSE,MSSA] Bail out early if there are no memory defs.

llvm/include/llvm/Analysis/MemorySSA.h

public:
   /// Verify that MemorySSA is self consistent (IE definitions dominate
   /// all uses, uses appear in the right places).  This is used by unit tests.
   void verifyMemorySSA(VerificationLevel = VerificationLevel::Fast) const;
 
   /// Used in various insertion functions to specify whether we are talking
   /// about the beginning or end of a block.
   enum InsertionPlace { Beginning, End, BeforeTerminator };
 
+  /// Returns true if there are no memory defs in the function.
+  bool empty_defs() const {
+    return PerBlockDefs.empty();
+  }
+
 protected:
   // Used by Memory SSA dumpers and wrapper pass
   friend class MemorySSAPrinterLegacyPass;
   friend class MemorySSAUpdater;
 
   void verifyOrderingDominationAndDefUses(
       Function &F, VerificationLevel = VerificationLevel::Fast) const;
   void verifyDominationNumbers(const Function &F) const;

llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp

   }
 };
 
 static bool eliminateDeadStores(Function &F, AliasAnalysis &AA, MemorySSA &MSSA,
                                 DominatorTree &DT, const TargetLibraryInfo &TLI,
                                 const LoopInfo &LI) {
   bool MadeChange = false;
 
+  if (MSSA.empty_defs())
+    return false;
+
   DSEState State(F, AA, MSSA, DT, TLI, LI);
   // For each store:
   for (unsigned I = 0; I < State.MemDefs.size(); I++) {
     MemoryDef *KillingDef = State.MemDefs[I];
     if (State.SkipStores.count(KillingDef))
       continue;
     Instruction *KillingI = KillingDef->getMemoryInst();