Index: include/llvm/Analysis/MemorySSAUpdater.h
===================================================================
--- include/llvm/Analysis/MemorySSAUpdater.h
+++ include/llvm/Analysis/MemorySSAUpdater.h
@@ -198,6 +198,12 @@
   /// deleted after this call.
   void removeBlocks(const SmallPtrSetImpl<BasicBlock *> &DeadBlocks);
 
+  /// Reset optimized for uses of a given instruction, if those uses have
+  /// MemoryAccesses. This should be called when the instruction is about to be
+  /// replaced, since its replacement may change the clobber (e.g., simplify to
+  /// undef).
+  void resetOptimizeUses(const Instruction *I);
+
   /// Get handle on MemorySSA.
   MemorySSA* getMemorySSA() const { return MSSA; }
 
Index: lib/Analysis/MemorySSAUpdater.cpp
===================================================================
--- lib/Analysis/MemorySSAUpdater.cpp
+++ lib/Analysis/MemorySSAUpdater.cpp
@@ -12,6 +12,7 @@
 //===----------------------------------------------------------------===//
 #include "llvm/Analysis/MemorySSAUpdater.h"
 #include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/SetVector.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/Analysis/MemorySSA.h"
 #include "llvm/IR/DataLayout.h"
@@ -607,6 +608,26 @@
   }
 }
 
+void MemorySSAUpdater::resetOptimizeUses(const Instruction *I) {
+  SmallSetVector<const Instruction *, 16> Worklist;
+  for (const auto *Usr : I->users())
+    if (auto *UsrI = dyn_cast<Instruction>(Usr))
+      Worklist.insert(UsrI);
+
+  for (unsigned It = 0; It < Worklist.size(); ++It) {
+    if (auto *MA = MSSA->getMemoryAccess(Worklist[It])) {
+      auto *MUD = cast<MemoryUseOrDef>(MA);
+      if (MUD->isOptimized())
+        MUD->resetOptimized();
+    } else {
+      for (const auto *Usr : Worklist[It]->users())
+        if (auto *UsrI = dyn_cast<Instruction>(Usr))
+          if (!Worklist.count(UsrI))
+            Worklist.insert(UsrI);
+    }
+  }
+}
+
 MemoryAccess *MemorySSAUpdater::createMemoryAccessInBB(
     Instruction *I, MemoryAccess *Definition, const BasicBlock *BB,
     MemorySSA::InsertionPlace Point) {