[llvm] [VPlan] Model branch cond to enter scalar epilogue in VPlan. (PR #92651)

[via llvm-commits]

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@@ -879,15 +922,41 @@ void VPlan::execute(VPTransformState *State) {
   State->CFG.ExitBB = State->CFG.PrevBB->getSingleSuccessor();
   BasicBlock *VectorPreHeader = State->CFG.PrevBB;
   State->Builder.SetInsertPoint(VectorPreHeader->getTerminator());
-  replaceVPBBWithIRVPBB(
-      cast<VPBasicBlock>(getVectorLoopRegion()->getSingleSuccessor()),
-      State->CFG.ExitBB);
 
   // Disconnect VectorPreHeader from ExitBB in both the CFG and DT.
   cast<BranchInst>(VectorPreHeader->getTerminator())->setSuccessor(0, nullptr);
   State->CFG.DTU.applyUpdates(
       {{DominatorTree::Delete, VectorPreHeader, State->CFG.ExitBB}});
 
+  // Replace regular VPBB's for the middle and scalar preheader blocks with
+  // VPIRBasicBlocks with VPIRBasicBlocks wrapping their IR blocks. The IR
+  // blocks are created during skeleton creation, so we can only create the
+  // VPIRBasicBlocks after legacy skeleton creation.
+  BasicBlock *MiddleBB = State->CFG.ExitBB;
+  VPBasicBlock *MiddleVPBB =
+      cast<VPBasicBlock>(getVectorLoopRegion()->getSingleSuccessor());
+  // Find the VPBB for the scalar preheader, relying on the current structure
+  // when creating the middle block and its successrs: if there's a single
+  // predecessor, it must be the scalar preheader. Otherwise, the second
+  // successor is the scalar preheader.
+  BasicBlock *ScalarPh = MiddleBB->getSingleSuccessor();
+  auto &MiddleSuccs = MiddleVPBB->getSuccessors();
+  assert((MiddleSuccs.size() == 1 || MiddleSuccs.size() == 2) &&
+         "middle block has unexpected successors");
+  VPBasicBlock *ScalarPhVPBB = cast<VPBasicBlock>(
+      MiddleSuccs.size() == 1 ? MiddleSuccs[0] : MiddleSuccs[1]);
+  assert(!isa<VPIRBasicBlock>(ScalarPhVPBB) &&
+         "scalar preheader cannot be wrapped already");
+  replaceVPBBWithIRVPBB(ScalarPhVPBB, ScalarPh);
+  replaceVPBBWithIRVPBB(MiddleVPBB, MiddleBB);
+
+  // Disconnect the middle block from its single successor (the scalar loop
+  // header) in both the CFG and DT. The branch will be created during VPlan
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ayalz wrote:

```suggestion
  // header) in both the CFG and DT. The branch will be recreated during VPlan
```

https://github.com/llvm/llvm-project/pull/92651