[LV] Remove more DT updates from legacy code path (NFCI).

Remove some legacy DT updates. Those should already be handled when
updating the DT during VPlan execution.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -2088,9 +2088,6 @@ class GeneratedRTChecks {
     Pred->getTerminator()->replaceSuccessorWith(LoopVectorPreHeader,
                                                 SCEVCheckBlock);
 
-    DT->addNewBlock(SCEVCheckBlock, Pred);
-    DT->changeImmediateDominator(LoopVectorPreHeader, SCEVCheckBlock);
-
     BranchInst &BI =
         *BranchInst::Create(Bypass, LoopVectorPreHeader, SCEVCheckCond);
     if (AddBranchWeights)
@@ -2115,8 +2112,6 @@ class GeneratedRTChecks {
     Pred->getTerminator()->replaceSuccessorWith(LoopVectorPreHeader,
                                                 MemCheckBlock);
 
-    DT->addNewBlock(MemCheckBlock, Pred);
-    DT->changeImmediateDominator(LoopVectorPreHeader, MemCheckBlock);
     MemCheckBlock->moveBefore(LoopVectorPreHeader);
 
     BranchInst &BI =
@@ -2556,13 +2551,9 @@ void InnerLoopVectorizer::emitIterationCountCheck(BasicBlock *Bypass) {
   }
 
   // Create new preheader for vector loop.
-  LoopVectorPreHeader =
-      SplitBlock(TCCheckBlock, TCCheckBlock->getTerminator(), DT, LI, nullptr,
-                 "vector.ph");
-
-  assert(DT->properlyDominates(DT->getNode(TCCheckBlock),
-                               DT->getNode(Bypass)->getIDom()) &&
-         "TC check is expected to dominate Bypass");
+  LoopVectorPreHeader = SplitBlock(TCCheckBlock, TCCheckBlock->getTerminator(),
+                                   static_cast<DominatorTree *>(nullptr), LI,
+                                   nullptr, "vector.ph");
 
   BranchInst &BI =
       *BranchInst::Create(Bypass, LoopVectorPreHeader, CheckMinIters);
@@ -7957,13 +7948,10 @@ EpilogueVectorizerMainLoop::emitIterationCountCheck(BasicBlock *Bypass,
 
   // Create new preheader for vector loop.
   LoopVectorPreHeader = SplitBlock(TCCheckBlock, TCCheckBlock->getTerminator(),
-                                   DT, LI, nullptr, "vector.ph");
+                                   static_cast<DominatorTree *>(nullptr), LI,
+                                   nullptr, "vector.ph");
 
   if (ForEpilogue) {
-    assert(DT->properlyDominates(DT->getNode(TCCheckBlock),
-                                 DT->getNode(Bypass)->getIDom()) &&
-           "TC check is expected to dominate Bypass");
-
     LoopBypassBlocks.push_back(TCCheckBlock);
 
     // Save the trip count so we don't have to regenerate it in the
@@ -8067,10 +8055,6 @@ EpilogueVectorizerEpilogueLoop::emitMinimumVectorEpilogueIterCountCheck(
 
   assert(EPI.TripCount &&
          "Expected trip count to have been saved in the first pass.");
-  assert(
-      (!isa<Instruction>(EPI.TripCount) ||
-       DT->dominates(cast<Instruction>(EPI.TripCount)->getParent(), Insert)) &&
-      "saved trip count does not dominate insertion point.");
   Value *TC = EPI.TripCount;
   IRBuilder<> Builder(Insert->getTerminator());
   Value *Count = Builder.CreateSub(TC, EPI.VectorTripCount, "n.vec.remaining");