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
@@ -607,6 +607,19 @@
   }
 }
 
+void MemorySSAUpdater::resetOptimizeUses(const Instruction *I) {
+  for (auto &U : I->uses()) {
+    auto *UsrI = dyn_cast<Instruction>(U.getUser());
+    if (!UsrI)
+      continue;
+    if (auto *MA = MSSA->getMemoryAccess(UsrI)) {
+      auto *MUD = cast<MemoryUseOrDef>(MA);
+      if (MUD->isOptimized())
+        MUD->resetOptimized();
+    }
+  }
+}
+
 MemoryAccess *MemorySSAUpdater::createMemoryAccessInBB(
     Instruction *I, MemoryAccess *Definition, const BasicBlock *BB,
     MemorySSA::InsertionPlace Point) {