Index: include/llvm/Analysis/MemorySSA.h
===================================================================
--- include/llvm/Analysis/MemorySSA.h
+++ include/llvm/Analysis/MemorySSA.h
@@ -793,6 +793,7 @@
   friend class MemorySSAPrinterLegacyPass;
   friend class MemorySSAUpdater;
 
+  void verifyPrevDefInPhis(Function &F) const;
   void verifyDefUses(Function &F) const;
   void verifyDomination(Function &F) const;
   void verifyOrdering(Function &F) const;
Index: lib/Analysis/MemorySSA.cpp
===================================================================
--- lib/Analysis/MemorySSA.cpp
+++ lib/Analysis/MemorySSA.cpp
@@ -1850,6 +1850,7 @@
   verifyDomination(F);
   verifyOrdering(F);
   verifyDominationNumbers(F);
+  verifyPrevDefInPhis(F);
   // Previously, the verification used to also verify that the clobberingAccess
   // cached by MemorySSA is the same as the clobberingAccess found at a later
   // query to AA. This does not hold true in general due to the current fragility
@@ -1862,6 +1863,33 @@
   // example, see test4 added in D51960.
 }
 
+void MemorySSA::verifyPrevDefInPhis(Function &F) const {
+  for (const BasicBlock &BB : F) {
+    if (MemoryPhi *Phi = getMemoryAccess(&BB)) {
+      for (unsigned I = 0, E = Phi->getNumIncomingValues(); I != E; ++I) {
+        auto *Pred = Phi->getIncomingBlock(I);
+        auto *IncAcc = Phi->getIncomingValue(I);
+        while (true) {
+          if (auto *DefList = getBlockDefs(Pred)) {
+            auto *LastAcc = &*(--DefList->end());
+            assert(LastAcc == IncAcc && "Incorrect incoming access into phi.");
+            (void)LastAcc;
+            break;
+          }
+          if (auto *DTNode = DT->getNode(Pred))
+            if (auto *IDom = DTNode->getIDom()) {
+              Pred = IDom->getBlock();
+              continue;
+            }
+          assert(IncAcc == getLiveOnEntryDef() && "Expected LoE inc");
+          (void)IncAcc;
+          break;
+        }
+      }
+    }
+  }
+}
+
 /// Verify that all of the blocks we believe to have valid domination numbers
 /// actually have valid domination numbers.
 void MemorySSA::verifyDominationNumbers(const Function &F) const {
Index: lib/Analysis/MemorySSAUpdater.cpp
===================================================================
--- lib/Analysis/MemorySSAUpdater.cpp
+++ lib/Analysis/MemorySSAUpdater.cpp
@@ -1031,7 +1031,7 @@
 
   // Now reinsert it into the IR and do whatever fixups needed.
   if (auto *MD = dyn_cast<MemoryDef>(What))
-    insertDef(MD);
+    insertDef(MD, true);
   else
     insertUse(cast<MemoryUse>(What));