Index: lib/Analysis/MemorySSAUpdater.cpp
===================================================================
--- lib/Analysis/MemorySSAUpdater.cpp
+++ lib/Analysis/MemorySSAUpdater.cpp
@@ -1184,6 +1184,14 @@
                                                Instruction *Start) {
   assert(From->getSinglePredecessor() == To &&
          "From block is expected to have a single predecessor (To).");
+  if (MemoryPhi *MP = MSSA->getMemoryAccess(From)) {
+    // `From` might have a trivial PHI with a single input. Optimize it
+    // now before `From` is removed.
+    assert(MP.getNumIncomingValues() == 1 && "From can only have one pred");
+    auto *ReplacedWith = tryRemoveTrivialPhi(MP);
+    assert(ReplacedWith != MP &&
+           "If there is a Phi it must be trivially removable");
+  }
   moveAllAccesses(From, To, Start);
   for (BasicBlock *Succ : successors(From))
     if (MemoryPhi *MPhi = MSSA->getMemoryAccess(Succ))
Index: lib/Transforms/Utils/BasicBlockUtils.cpp
===================================================================
--- lib/Transforms/Utils/BasicBlockUtils.cpp
+++ lib/Transforms/Utils/BasicBlockUtils.cpp
@@ -227,18 +227,24 @@
     Updates.push_back({DominatorTree::Delete, PredBB, BB});
   }
 
-  if (MSSAU)
-    MSSAU->moveAllAfterMergeBlocks(BB, PredBB, &*(BB->begin()));
-
-  // Delete the unconditional branch from the predecessor...
-  PredBB->getInstList().pop_back();
+  // We need to update MemorySSA if MSSAU is provided. MemorySSAUpdater expects
+  // a specifc state when notified: `PredBB` still branches to `BB`, but all
+  // instructions were moved from `BB` to `PredBB` and `BB` is now an empty
+  // block. Splice the instructions, but keep the terminators intact.
+  Instruction *PredBBTerm = PredBB->getTerminator();
+  Instruction *BBTerm = BB->getTerminator();
+  Instruction *FirstBBInst = &*(BB->begin());
+  PredBB->getInstList().splice(PredBBTerm->getIterator(), BB->getInstList(),
+                               BB->begin(), BBTerm->getIterator());
 
-  // Make all PHI nodes that referred to BB now refer to Pred as their
-  // source...
-  BB->replaceAllUsesWith(PredBB);
+  if (MSSAU)
+    MSSAU->moveAllAfterMergeBlocks(BB, PredBB, FirstBBInst);
 
-  // Move all definitions in the successor to the predecessor...
-  PredBB->getInstList().splice(PredBB->end(), BB->getInstList());
+  // Finish updating the CFG after MSSAU is notified.
+  BB->replaceSuccessorsPhiUsesWith(PredBB);
+  BBTerm->removeFromParent();
+  BBTerm->insertBefore(PredBBTerm);
+  PredBBTerm->eraseFromParent();
   new UnreachableInst(BB->getContext(), BB);
 
   // Eliminate duplicate dbg.values describing the entry PHI node post-splice.
Index: test/Transforms/LoopSimplifyCFG/unswitch_simplify_mssa_update.ll
===================================================================
--- /dev/null
+++ test/Transforms/LoopSimplifyCFG/unswitch_simplify_mssa_update.ll
@@ -0,0 +1,32 @@
+; RUN: opt -passes='loop-mssa(unswitch,simplify-cfg),verify<memoryssa>' -S < %s | FileCheck %s
+
+declare void @a()
+declare void @b()
+declare void @x()
+
+; CHECK: test_unswitch_simplify
+define void @test_unswitch_simplify(i1* %ptr, i1 %cond, i1 %f, i32 %n) {
+entry:
+  br i1 %f, label %loop_begin, label %loop_exit
+
+loop_begin:
+  %iv = phi i32 [0, %entry], [%iv.next, %loop_latch]
+  call void @x()
+  br i1 %cond, label %loop_a, label %loop_b
+
+loop_a:
+  call void @a()
+  br label %loop_latch
+
+loop_b:
+  call void @b()
+  br label %loop_latch
+
+loop_latch:
+  %iv.next = add i32 %iv, 1
+  %v = icmp eq i32 %iv.next, %n
+  br i1 %v, label %loop_begin, label %loop_exit
+
+loop_exit:
+  ret void
+}
\ No newline at end of file