diff --git a/llvm/lib/Transforms/Scalar/LoopUnrollAndJamPass.cpp b/llvm/lib/Transforms/Scalar/LoopUnrollAndJamPass.cpp
--- a/llvm/lib/Transforms/Scalar/LoopUnrollAndJamPass.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopUnrollAndJamPass.cpp
@@ -450,7 +450,6 @@
   internal::appendLoopsToWorklist(reverse(LI), Worklist);
   while (!Worklist.empty()) {
     Loop *L = Worklist.pop_back_val();
-    formLCSSA(*L, DT, &LI, &SE);
     LoopUnrollResult Result =
         tryToUnrollAndJamLoop(L, DT, &LI, SE, TTI, AC, DI, ORE, OptLevel);
     if (Result != LoopUnrollResult::Unmodified)
diff --git a/llvm/lib/Transforms/Utils/LoopUnrollAndJam.cpp b/llvm/lib/Transforms/Utils/LoopUnrollAndJam.cpp
--- a/llvm/lib/Transforms/Utils/LoopUnrollAndJam.cpp
+++ b/llvm/lib/Transforms/Utils/LoopUnrollAndJam.cpp
@@ -284,8 +284,7 @@
 
   // Move any instructions from fore phi operands from AftBlocks into Fore.
   moveHeaderPhiOperandsToForeBlocks(
-      Header, LatchBlock, SubLoop->getLoopPreheader()->getTerminator(),
-      AftBlocks);
+      Header, LatchBlock, ForeBlocksLast[0]->getTerminator(), AftBlocks);
 
   // The current on-the-fly SSA update requires blocks to be processed in
   // reverse postorder so that LastValueMap contains the correct value at each
@@ -312,7 +311,7 @@
 
   // Copy all blocks
   for (unsigned It = 1; It != Count; ++It) {
-    std::vector<BasicBlock *> NewBlocks;
+    SmallVector<BasicBlock *, 4> NewBlocks;
     // Maps Blocks[It] -> Blocks[It-1]
     DenseMap<Value *, Value *> PrevItValueMap;
 
@@ -379,9 +378,9 @@
     }
 
     // Remap all instructions in the most recent iteration
+    remapInstructionsInBlocks(NewBlocks, LastValueMap);
     for (BasicBlock *NewBlock : NewBlocks) {
       for (Instruction &I : *NewBlock) {
-        ::remapInstruction(&I, LastValueMap);
         if (auto *II = dyn_cast<IntrinsicInst>(&I))
           if (II->getIntrinsicID() == Intrinsic::assume)
             AC->registerAssumption(II);
@@ -447,8 +446,8 @@
   // Update ForeBlocks successors and phi nodes
   BranchInst *ForeTerm =
       cast<BranchInst>(ForeBlocksLast.back()->getTerminator());
-  BasicBlock *Dest = SubLoopBlocksFirst[0];
-  ForeTerm->setSuccessor(0, Dest);
+  assert(ForeTerm->getNumSuccessors() == 1 && "Expecting one successor");
+  ForeTerm->setSuccessor(0, SubLoopBlocksFirst[0]);
 
   if (CompletelyUnroll) {
     while (PHINode *Phi = dyn_cast<PHINode>(ForeBlocksFirst[0]->begin())) {
@@ -465,8 +464,8 @@
     // Remap ForeBlock successors from previous iteration to this
     BranchInst *ForeTerm =
         cast<BranchInst>(ForeBlocksLast[It - 1]->getTerminator());
-    BasicBlock *Dest = ForeBlocksFirst[It];
-    ForeTerm->setSuccessor(0, Dest);
+    assert(ForeTerm->getNumSuccessors() == 1 && "Expecting one successor");
+    ForeTerm->setSuccessor(0, ForeBlocksFirst[It]);
   }
 
   // Subloop successors and phis
@@ -495,12 +494,14 @@
   }
 
   // Aft blocks successors and phis
-  BranchInst *Term = cast<BranchInst>(AftBlocksLast.back()->getTerminator());
+  BranchInst *AftTerm = cast<BranchInst>(AftBlocksLast.back()->getTerminator());
   if (CompletelyUnroll) {
-    BranchInst::Create(LoopExit, Term);
-    Term->eraseFromParent();
+    BranchInst::Create(LoopExit, AftTerm);
+    AftTerm->eraseFromParent();
   } else {
-    Term->setSuccessor(!ContinueOnTrue, ForeBlocksFirst[0]);
+    AftTerm->setSuccessor(!ContinueOnTrue, ForeBlocksFirst[0]);
+    assert(AftTerm->getSuccessor(ContinueOnTrue) == LoopExit &&
+           "Expecting the ContinueOnTrue successor of AftTerm to be LoopExit");
   }
   updatePHIBlocks(AftBlocksFirst[0], SubLoopBlocksLast[0],
                   SubLoopBlocksLast.back());
@@ -518,24 +519,16 @@
     movePHIs(AftBlocksFirst[It], AftBlocksFirst[0]);
   }
 
-  DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy);
   // Dominator Tree. Remove the old links between Fore, Sub and Aft, adding the
   // new ones required.
   if (Count != 1) {
-    SmallVector<DominatorTree::UpdateType, 4> DTUpdates;
-    DTUpdates.emplace_back(DominatorTree::UpdateKind::Delete, ForeBlocksLast[0],
-                           SubLoopBlocksFirst[0]);
-    DTUpdates.emplace_back(DominatorTree::UpdateKind::Delete,
-                           SubLoopBlocksLast[0], AftBlocksFirst[0]);
-
-    DTUpdates.emplace_back(DominatorTree::UpdateKind::Insert,
-                           ForeBlocksLast.back(), SubLoopBlocksFirst[0]);
-    DTUpdates.emplace_back(DominatorTree::UpdateKind::Insert,
-                           SubLoopBlocksLast.back(), AftBlocksFirst[0]);
-    DTU.applyUpdatesPermissive(DTUpdates);
+    DT->changeImmediateDominator(SubLoopBlocksFirst[0], ForeBlocksLast.back());
+    DT->changeImmediateDominator(AftBlocksFirst[0], SubLoopBlocksLast.back());
+    DT->changeImmediateDominator(LoopExit, AftBlocksLast.back());
   }
 
   // Merge adjacent basic blocks, if possible.
+  DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy);
   SmallPtrSet<BasicBlock *, 16> MergeBlocks;
   MergeBlocks.insert(ForeBlocksLast.begin(), ForeBlocksLast.end());
   MergeBlocks.insert(SubLoopBlocksLast.begin(), SubLoopBlocksLast.end());
@@ -568,21 +561,26 @@
   NumCompletelyUnrolledAndJammed += CompletelyUnroll;
   ++NumUnrolledAndJammed;
 
+  // Update LoopInfo if the loop is completely removed.
+  if (CompletelyUnroll)
+    LI->erase(L);
+
 #ifndef NDEBUG
   // We shouldn't have done anything to break loop simplify form or LCSSA.
-  Loop *OuterL = L->getParentLoop();
-  Loop *OutestLoop = OuterL ? OuterL : (!CompletelyUnroll ? L : SubLoop);
+  Loop *OutestLoop = SubLoop->getParentLoop()
+                         ? SubLoop->getParentLoop()->getParentLoop()
+                               ? SubLoop->getParentLoop()->getParentLoop()
+                               : SubLoop->getParentLoop()
+                         : SubLoop;
+  assert(DT->verify());
+  LI->verify(*DT);
   assert(OutestLoop->isRecursivelyLCSSAForm(*DT, *LI));
   if (!CompletelyUnroll)
     assert(L->isLoopSimplifyForm());
   assert(SubLoop->isLoopSimplifyForm());
-  assert(DT->verify());
+  SE->verify();
 #endif
 
-  // Update LoopInfo if the loop is completely removed.
-  if (CompletelyUnroll)
-    LI->erase(L);
-
   return CompletelyUnroll ? LoopUnrollResult::FullyUnrolled
                           : LoopUnrollResult::PartiallyUnrolled;
 }