Index: include/llvm/Analysis/MemorySSAUpdater.h
===================================================================
--- include/llvm/Analysis/MemorySSAUpdater.h
+++ include/llvm/Analysis/MemorySSAUpdater.h
@@ -132,7 +132,11 @@
   void
   wireOldPredecessorsToNewImmediatePredecessor(BasicBlock *Old, BasicBlock *New,
                                                ArrayRef<BasicBlock *> Preds);
-
+  /// `ToReplace` used to be a predecessor of BB. `ReplaceWith` is the new
+  /// predecessor. Update Phi in BB to replace incoming block `ToReplace` with
+  /// `ReplaceWith`.
+  void replacePhiIncomingBlock(BasicBlock *BB, BasicBlock *ToReplace,
+                               BasicBlock *ReplaceWith);
   // The below are utility functions. Other than creation of accesses to pass
   // to insertDef, and removeAccess to remove accesses, you should generally
   // not attempt to update memoryssa yourself. It is very non-trivial to get
Index: lib/Analysis/MemorySSAUpdater.cpp
===================================================================
--- lib/Analysis/MemorySSAUpdater.cpp
+++ lib/Analysis/MemorySSAUpdater.cpp
@@ -392,6 +392,16 @@
   }
 }
 
+void MemorySSAUpdater::replacePhiIncomingBlock(BasicBlock *BB,
+                                               BasicBlock *ToReplace,
+                                               BasicBlock *ReplaceWith) {
+  if (MemoryPhi *MPhi = MSSA->getMemoryAccess(BB)) {
+    unsigned I = MPhi->getBasicBlockIndex(ToReplace);
+    assert((I >= 0) && "MemoryPhi position out of bounds");
+    MPhi->setIncomingBlock(I, ReplaceWith);
+  }
+}
+
 // Move What before Where in the MemorySSA IR.
 template <class WhereType>
 void MemorySSAUpdater::moveTo(MemoryUseOrDef *What, BasicBlock *BB,