Ensure reassociateBlocks leaves Old and New in a decent state

* Since reassociateBlocks is a public interface we should leave
the Old and New blocks in a usable state.

Author: david-arm

llvm/lib/Transforms/Vectorize/VPlan.cpp

@@ -1006,10 +1006,7 @@ static void replaceVPBBWithIRVPBB(VPBasicBlock *VPBB, BasicBlock *IRBB) {
   }
 
   VPBlockUtils::reassociateBlocks(VPBB, IRVPBB);
-  IRVPBB->setPredecessors(VPBB->getPredecessors());
-  IRVPBB->setSuccessors(VPBB->getSuccessors());
 
-  // Any successor/predecessor lists will be cleared on deletion.
   delete VPBB;
 }
 

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -3889,6 +3889,10 @@ class VPBlockUtils {
       Pred->replaceSuccessor(Old, New);
     for (auto *Succ : to_vector(Old->getSuccessors()))
       Succ->replacePredecessor(Old, New);
+    New->setPredecessors(Old->getPredecessors());
+    New->setSuccessors(Old->getSuccessors());
+    Old->clearPredecessors();
+    Old->clearSuccessors();
   }
 
   /// Return an iterator range over \p Range which only includes \p BlockTy