[VPlan] Iterate over blocks in VPlan::execute in RPOT (NFC).

This prepares for more complex CFGs in VPlan, as in
    #114292
    #112138

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlan.cpp

@@ -1033,8 +1033,11 @@ void VPlan::execute(VPTransformState *State) {
   State->CFG.DTU.applyUpdates(
       {{DominatorTree::Delete, ScalarPh, ScalarPh->getSingleSuccessor()}});
 
-  // Generate code in the loop pre-header and body.
-  for (VPBlockBase *Block : vp_depth_first_shallow(Entry))
+  ReversePostOrderTraversal<VPBlockShallowTraversalWrapper<VPBlockBase *>> RPOT(
+      Entry);
+  // Generate code for the VPlan, in parts of the vector skeleton, loop body and
+  // successor blocks including the middle, exit and scalar preheader blocks.
+  for (VPBlockBase *Block : RPOT)
     Block->execute(State);
 
   VPBasicBlock *LatchVPBB = getVectorLoopRegion()->getExitingBasicBlock();