[VPlan] Perform DT expensive input DT verification earlier (NFC).

After 6c8f41d, DT adjustments for the skeleton are applied as VPBBs
are executed. Move input DT verification up before starting to execute
any VPBBs to avoid checking DT while the CFG and DT are in an incomplete
state.

This fixes a number of verification failures with expensive checks
enabled, including
https://lab.llvm.org/buildbot/#/builders/16/builds/10584

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -7774,6 +7774,10 @@ DenseMap<const SCEV *, Value *> LoopVectorizationPlanner::executePlan(
   VPTransformState State(&TTI, BestVF, BestUF, LI, DT, ILV.Builder, &ILV,
                          &BestVPlan, Legal->getWidestInductionType());
 
+#ifdef EXPENSIVE_CHECKS
+  assert(DT->verify(DominatorTree::VerificationLevel::Fast));
+#endif
+
   // 0. Generate SCEV-dependent code into the preheader, including TripCount,
   // before making any changes to the CFG.
   if (!BestVPlan.getPreheader()->empty()) {
@@ -7794,10 +7798,6 @@ DenseMap<const SCEV *, Value *> LoopVectorizationPlanner::executePlan(
   if (VectorizingEpilogue)
     VPlanTransforms::removeDeadRecipes(BestVPlan);
 
-#ifdef EXPENSIVE_CHECKS
-  assert(DT->verify(DominatorTree::VerificationLevel::Fast));
-#endif
-
   // Only use noalias metadata when using memory checks guaranteeing no overlap
   // across all iterations.
   const LoopAccessInfo *LAI = ILV.Legal->getLAI();