[llvm] [NFC][LoopVectorize] Make replaceVPBBWithIRVPBB more efficient (PR #111514)

[Paul Walker via llvm-commits]

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@@ -1005,12 +1005,14 @@ static void replaceVPBBWithIRVPBB(VPBasicBlock *VPBB, BasicBlock *IRBB) {
     R.moveBefore(*IRVPBB, IRVPBB->end());
   }
   VPBlockBase *PredVPBB = VPBB->getSinglePredecessor();
-  VPBlockUtils::disconnectBlocks(PredVPBB, VPBB);
-  VPBlockUtils::connectBlocks(PredVPBB, IRVPBB);
-  for (auto *Succ : to_vector(VPBB->getSuccessors())) {
-    VPBlockUtils::connectBlocks(IRVPBB, Succ);
-    VPBlockUtils::disconnectBlocks(VPBB, Succ);
-  }
+  PredVPBB->replaceSuccessor(VPBB, IRVPBB);
+  IRVPBB->setPredecessors({PredVPBB});
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paulwalker-arm wrote:

Can you be sure that for all future uses of this function `IRVPBB`'s predecessors and successors can be replaced?  Would it be safer to append them as was done previously?

>From a structural point of view `VPBlockUtils` looks to be the place for managing VPBB linkage (I see a range of insert methods along with the (dis)connect method you're replacing) so in keeping with this perhaps add `VPBlockUtils::reassociateBlock(Keep, Old, New)` that calls onto `replaceSuccessor` and `replacePredecessor`.

https://github.com/llvm/llvm-project/pull/111514