[LV] Remove now obsolete DT updates of scalar exit block.

Remove manual DT updates of scalar exit blocks during legacy skeleton
creation, as they are not needed after 99d6c6d.

This fixes DT verification failures with expensive checks, including
https://lab.llvm.org/buildbot/#/builders/16/builds/1270.

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -2873,12 +2873,6 @@ void InnerLoopVectorizer::emitIterationCountCheck(BasicBlock *Bypass) {
 
   // Update dominator for Bypass & LoopExit (if needed).
   DT->changeImmediateDominator(Bypass, TCCheckBlock);
-  if (!Cost->requiresScalarEpilogue(VF.isVector()))
-    // If there is an epilogue which must run, there's no edge from the
-    // middle block to exit blocks  and thus no need to update the immediate
-    // dominator of the exit blocks.
-    DT->changeImmediateDominator(LoopExitBlock, TCCheckBlock);
-
   BranchInst &BI =
       *BranchInst::Create(Bypass, LoopVectorPreHeader, CheckMinIters);
   if (hasBranchWeightMD(*OrigLoop->getLoopLatch()->getTerminator()))
@@ -7611,14 +7605,8 @@ EpilogueVectorizerMainLoop::emitIterationCountCheck(BasicBlock *Bypass,
                                  DT->getNode(Bypass)->getIDom()) &&
            "TC check is expected to dominate Bypass");
 
-    // Update dominator for Bypass & LoopExit.
+    // Update dominator for Bypass.
     DT->changeImmediateDominator(Bypass, TCCheckBlock);
-    if (!Cost->requiresScalarEpilogue(EPI.EpilogueVF.isVector()))
-      // For loops with multiple exits, there's no edge from the middle block
-      // to exit blocks (as the epilogue must run) and thus no need to update
-      // the immediate dominator of the exit blocks.
-      DT->changeImmediateDominator(LoopExitBlock, TCCheckBlock);
-
     LoopBypassBlocks.push_back(TCCheckBlock);
 
     // Save the trip count so we don't have to regenerate it in the