Revert "[VPlan] Remove unnecessary DomTreeUpdater flush (NFC)." (#144758)

This reverts commit 2e33734.

Causes breakages internally, will post reproducer later.

Author: aeubanks

llvm/lib/Transforms/Vectorize/VPlan.cpp

@@ -1015,6 +1015,8 @@ void VPlan::execute(VPTransformState *State) {
   for (VPBlockBase *Block : RPOT)
     Block->execute(State);
 
+  State->CFG.DTU.flush();
+
   VPBasicBlock *Header = vputils::getFirstLoopHeader(*this, State->VPDT);
   if (!Header)
     return;