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
@@ -363,14 +363,11 @@
     // place, compute IDF and place phis.
     SmallPtrSet<BasicBlock *, 2> DefiningBlocks;
 
-    // If this is the last Def in the block, also compute IDF based on MD, since
-    // this may a new Def added, and we may need additional Phis.
-    auto Iter = MD->getDefsIterator();
-    ++Iter;
-    auto IterEnd = MSSA->getBlockDefs(MD->getBlock())->end();
-    if (Iter == IterEnd)
-      DefiningBlocks.insert(MD->getBlock());
-
+    // If this is the last Def in the block, we may need additional Phis.
+    // Compute IDF in all cases, as renaming needs to be done even when MD is
+    // not the last access, because it can introduce a new access past which a
+    // previous access was optimized; that access needs to be reoptimized.
+    DefiningBlocks.insert(MD->getBlock());
     for (const auto &VH : InsertedPHIs)
       if (const auto *RealPHI = cast_or_null<MemoryPhi>(VH))
         DefiningBlocks.insert(RealPHI->getBlock());