[VPlan] Use replaceSuccessor/replacePredecessor in insertBlock (NFC).

Use replaceSuccessor/replacePredecessor in
insertBlockAfter/insertBlockBefore. This preserves the predecessor
order, which in turns is needed to not invalidate existing phi recipes.

At the moment this is NFC, but enables additional uses in the future.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlanUtils.h

@@ -90,9 +90,10 @@ class VPBlockUtils {
     NewBlock->setParent(BlockPtr->getParent());
     SmallVector<VPBlockBase *> Succs(BlockPtr->successors());
     for (VPBlockBase *Succ : Succs) {
-      disconnectBlocks(BlockPtr, Succ);
-      connectBlocks(NewBlock, Succ);
+      Succ->replacePredecessor(BlockPtr, NewBlock);
+      NewBlock->appendSuccessor(Succ);
     }
+    BlockPtr->clearSuccessors();
     connectBlocks(BlockPtr, NewBlock);
   }
 
@@ -106,9 +107,10 @@ class VPBlockUtils {
            "Can't insert new block with predecessors or successors.");
     NewBlock->setParent(BlockPtr->getParent());
     for (VPBlockBase *Pred : to_vector(BlockPtr->predecessors())) {
-      disconnectBlocks(Pred, BlockPtr);
-      connectBlocks(Pred, NewBlock);
+      Pred->replaceSuccessor(BlockPtr, NewBlock);
+      NewBlock->appendPredecessor(Pred);
     }
+    BlockPtr->clearPredecessors();
     connectBlocks(NewBlock, BlockPtr);
   }