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

[via llvm-commits]

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@@ -443,13 +443,30 @@ VPBasicBlock::createEmptyBasicBlock(VPTransformState::CFGState &CFG) {
 }
 
 void VPIRBasicBlock::execute(VPTransformState *State) {
-  assert(getHierarchicalPredecessors().empty() &&
-         "VPIRBasicBlock cannot have predecessors at the moment");
   assert(getHierarchicalSuccessors().empty() &&
          "VPIRBasicBlock cannot have successors at the moment");
 
   State->Builder.SetInsertPoint(getIRBasicBlock()->getTerminator());
   executeRecipes(State, getIRBasicBlock());
+
+  for (VPBlockBase *PredVPBlock : getHierarchicalPredecessors()) {
+    VPBasicBlock *PredVPBB = PredVPBlock->getExitingBasicBlock();
+    auto &PredVPSuccessors = PredVPBB->getHierarchicalSuccessors();
+    BasicBlock *PredBB = State->CFG.VPBB2IRBB[PredVPBB];
+
+    assert(PredBB && "Predecessor basic-block not found building successor.");
+    auto *PredBBTerminator = PredBB->getTerminator();
+    LLVM_DEBUG(dbgs() << "LV: draw edge from" << PredBB->getName() << '\n');
+
+    auto *TermBr = cast<BranchInst>(PredBBTerminator);
+    // Set each forward successor here when it is created, excluding
+    // backedges. A backward successor is set when the branch is created.
----------------
ayalz wrote:

```suggestion
    BasicBlock *PredBB = State->CFG.VPBB2IRBB[PredVPBB];
    assert(PredBB && "Predecessor basic-block not found building successor.");
    LLVM_DEBUG(dbgs() << "LV: draw edge from" << PredBB->getName() << '\n');

    auto *PredBBTerminator = PredBB->getTerminator();
    auto *TermBr = cast<BranchInst>(PredBBTerminator);
    // Set each forward successor here when it is created, excluding
    // backedges. A backward successor is set when the branch is created.
    auto &PredVPSuccessors = PredVPBB->getHierarchicalSuccessors();
```

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