diff --git a/llvm/lib/Analysis/MemorySSAUpdater.cpp b/llvm/lib/Analysis/MemorySSAUpdater.cpp
--- a/llvm/lib/Analysis/MemorySSAUpdater.cpp
+++ b/llvm/lib/Analysis/MemorySSAUpdater.cpp
@@ -342,6 +342,8 @@
 
   SmallVector<WeakVH, 8> FixupList(InsertedPHIs.begin(), InsertedPHIs.end());
 
+  SmallSet<WeakVH, 8> ExistingPhis;
+
   // Remember the index where we may insert new phis.
   unsigned NewPhiIndex = InsertedPHIs.size();
   if (!DefBeforeSameBlock) {
@@ -382,6 +384,8 @@
       if (!MPhi) {
         MPhi = MSSA->createMemoryPhi(BBIDF);
         NewInsertedPHIs.push_back(MPhi);
+      } else {
+        ExistingPhis.insert(MPhi);
       }
       // Add the phis created into the IDF blocks to NonOptPhis, so they are not
       // optimized out as trivial by the call to getPreviousDefFromEnd below.
@@ -447,6 +451,13 @@
       if (Phi)
         MSSA->renamePass(Phi->getBlock(), nullptr, Visited);
     }
+    // Existing Phi blocks may need renaming too, if an access was previously
+    // optimized and the inserted Defs "covers" the Optimized value.
+    for (auto &MP : ExistingPhis) {
+      MemoryPhi *Phi = dyn_cast_or_null<MemoryPhi>(MP);
+      if (Phi)
+        MSSA->renamePass(Phi->getBlock(), nullptr, Visited);
+    }
   }
 }
 
@@ -1322,6 +1333,7 @@
     // Note: We assume MemorySSA is not used in metadata since it's not really
     // part of the IR.
 
+    assert(NewDefTarget != MA && "Going into an infinite loop");
     while (!MA->use_empty()) {
       Use &U = *MA->use_begin();
       if (auto *MUD = dyn_cast<MemoryUseOrDef>(U.getUser()))
diff --git a/llvm/test/Analysis/MemorySSA/pr45927.ll b/llvm/test/Analysis/MemorySSA/pr45927.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/Analysis/MemorySSA/pr45927.ll
@@ -0,0 +1,73 @@
+; RUN: opt -disable-output -loop-simplify -lcssa -licm -print-memoryssa < %s 2>&1 | FileCheck %s
+; RUN: opt -disable-output -aa-pipeline=basic-aa -passes='loop-mssa(licm),print<memoryssa>' < %s 2>&1 | FileCheck %s
+
+
+@a = external dso_local global i16, align 1
+@c = external dso_local global i16, align 1
+
+; CHECK-LABEL: @main()
+
+; CHECK: entry:
+; CHECK-NEXT: %res.addr.i = alloca i16
+; CHECK-NEXT: ; MemoryUse(liveOnEntry)
+; CHECK-NEXT: %c.promoted = load i16, i16* @c
+; CHECK-NEXT: br label %for.cond.i
+
+; CHECK: for.cond.i:
+; CHECK-NEXT: ; [[NO5:.*]] = MemoryPhi({entry,liveOnEntry},{f.exit.i,[[NO5]]})
+; CHECK-NEXT: %inc.i1 = phi i16 [ %inc.i, %f.exit.i ], [ %c.promoted, %entry ]
+; CHECK-NEXT: %inc.i = add nsw i16 %inc.i1, 1
+; CHECK-NEXT: br i1 false, label %f.exit.thread.i, label %f.exit.i
+
+; CHECK: f.exit.thread.i:
+; CHECK-NEXT: %inc.i.lcssa = phi i16 [ %inc.i, %for.cond.i ]
+; CHECK-NEXT: ; [[NO6:.*]] = MemoryDef([[NO5]])
+; CHECK-NEXT: store i16 %inc.i.lcssa, i16* @c, align 1
+; CHECK-NEXT: ; [[NO2:.*]] = MemoryDef([[NO6]])
+; CHECK-NEXT: store i16 1, i16* @a, align 1
+; CHECK-NEXT: ; MemoryUse([[NO2]])
+; CHECK-NEXT: %tmp2 = load i16, i16* @c, align 1
+; CHECK-NEXT: br label %g.exit
+
+; CHECK: f.exit.i
+; CHECK-NEXT: br i1 false, label %g.exit.loopexit, label %for.cond.i
+
+; CHECK: g.exit.loopexit:
+; CHECK-NEXT: %inc.i.lcssa2 = phi i16 [ %inc.i, %f.exit.i ]
+; CHECK-NEXT: ; [[NO7:.*]] = MemoryDef([[NO5]])
+; CHECK-NEXT: store i16 %inc.i.lcssa2, i16* @c, align 1
+; CHECK-NEXT: br label %g.exit
+
+; CHECK: g.exit
+; CHECK-NEXT: ; [[NO4:.*]] = MemoryPhi({f.exit.thread.i,[[NO2]]},{g.exit.loopexit,[[NO7]]})
+; CHECK-NEXT: ; MemoryUse([[NO4]])
+; CHECK-NEXT:  %tmp1 = load i16, i16* @c, align 1
+; CHECK-NEXT: ; [[NO3:.*]] = MemoryDef([[NO4]])
+; CHECK-NEXT:  store i16 %tmp1, i16* %res.addr.i, align 1
+; CHECK-NEXT:  ret void
+
+define dso_local void @main() {
+entry:
+  %res.addr.i = alloca i16, align 1
+  br label %for.cond.i
+
+for.cond.i:                                       ; preds = %f.exit.i, %entry
+  %tmp0 = load i16, i16* @c, align 1
+  %inc.i = add nsw i16 %tmp0, 1
+  store i16 %inc.i, i16* @c, align 1
+  br i1 false, label %f.exit.thread.i, label %f.exit.i
+
+f.exit.thread.i:                                  ; preds = %for.cond.i
+  store i16 1, i16* @a, align 1
+  %tmp2 = load i16, i16* @c, align 1
+  br label %g.exit
+
+f.exit.i:                                         ; preds = %for.cond.i
+  br i1 false, label %g.exit, label %for.cond.i
+
+g.exit:                                           ; preds = %f.exit.i, %f.exit.thread.i
+  %tmp1 = load i16, i16* @c, align 1
+  store i16 %tmp1, i16* %res.addr.i, align 1
+  ret void
+}
+