Address review comments

* Add new VPBlockUtils::reassociateBlocks that replaces all
associations to Old with New.
* Removed code to clear successors/predecessors in the deleted
block.

Author: david-arm

llvm/lib/Transforms/Vectorize/VPlan.cpp

@@ -1004,15 +1004,12 @@ static void replaceVPBBWithIRVPBB(VPBasicBlock *VPBB, BasicBlock *IRBB) {
     assert(!R.isPhi() && "Tried to move phi recipe to end of block");
     R.moveBefore(*IRVPBB, IRVPBB->end());
   }
-  VPBlockBase *PredVPBB = VPBB->getSinglePredecessor();
-  PredVPBB->replaceSuccessor(VPBB, IRVPBB);
-  IRVPBB->setPredecessors({PredVPBB});
-  for (auto *Succ : to_vector(VPBB->getSuccessors()))
-    Succ->replacePredecessor(VPBB, IRVPBB);
+
+  VPBlockUtils::reassociateBlocks(VPBB, IRVPBB);
+  IRVPBB->setPredecessors(VPBB->getPredecessors());
   IRVPBB->setSuccessors(VPBB->getSuccessors());
 
-  VPBB->clearSuccessors();
-  VPBB->clearPredecessors();
+  // Any successor/predecessor lists will be cleared on deletion.
   delete VPBB;
 }
 

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -3882,6 +3882,15 @@ class VPBlockUtils {
     To->removePredecessor(From);
   }
 
+  /// Reassociate all the blocks connected to \p Old so that they now point to
+  /// \p New.
+  static void reassociateBlocks(VPBlockBase *Old, VPBlockBase *New) {
+    for (auto *Pred : to_vector(Old->getPredecessors()))
+      Pred->replaceSuccessor(Old, New);
+    for (auto *Succ : to_vector(Old->getSuccessors()))
+      Succ->replacePredecessor(Old, New);
+  }
+
   /// Return an iterator range over \p Range which only includes \p BlockTy
   /// blocks. The accesses are casted to \p BlockTy.
   template <typename BlockTy, typename T>