[NFC][LoopVectorize] Make replaceVPBBWithIRVPBB more efficient

In replaceVPBBWithIRVPBB we spend time erasing and appending
predecessors and successors from a list, when all we really have
to do is replace the old with the new. Not only is this more
efficient, but it also preserves the ordering of successors and
predecessors. This is something which may become important for
vectorising early exit loops (see PR #88385), since a
VPIRInstruction is the wrapper for a live-out phi with extra
operands that map to the incoming block according to the block's
predecessor.

Author: david-arm

llvm/lib/Transforms/Vectorize/VPlan.cpp

@@ -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});
+  for (auto *Succ : to_vector(VPBB->getSuccessors()))
+    Succ->replacePredecessor(VPBB, IRVPBB);
+  IRVPBB->setSuccessors(VPBB->getSuccessors());
+
+  VPBB->clearSuccessors();
+  VPBB->clearPredecessors();
   delete VPBB;
 }
 

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -556,6 +556,26 @@ class VPBlockBase {
     return getEnclosingBlockWithPredecessors()->getSinglePredecessor();
   }
 
+  /// This function replaces one predecessor with another, useful when
+  /// trying to replace an old block in the CFG with a new one.
+  void replacePredecessor(VPBlockBase *Old, VPBlockBase *New) {
+    auto I = find(Predecessors, Old);
+    assert(I != Predecessors.end());
+    assert(Old->getParent() == New->getParent() &&
+           "replaced predecessor must have the same parent");
+    *I = New;
+  }
+
+  /// This function replaces one successor with another, useful when
+  /// trying to replace an old block in the CFG with a new one.
+  void replaceSuccessor(VPBlockBase *Old, VPBlockBase *New) {
+    auto I = find(Successors, Old);
+    assert(I != Successors.end());
+    assert(Old->getParent() == New->getParent() &&
+           "replaced successor must have the same parent");
+    *I = New;
+  }
+
   /// Set a given VPBlockBase \p Successor as the single successor of this
   /// VPBlockBase. This VPBlockBase is not added as predecessor of \p Successor.
   /// This VPBlockBase must have no successors.