[llvm] [VPlan] Introduce ExitPhi VPInstruction, use to create phi for FOR. (PR #94760)

[via llvm-commits]

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@@ -593,13 +595,31 @@ Value *VPInstruction::generatePerPart(VPTransformState &State, unsigned Part) {
     Value *Addend = State.get(getOperand(1), Part, /* IsScalar */ true);
     return Builder.CreatePtrAdd(Ptr, Addend, Name);
   }
+  case VPInstruction::ExitPhi: {
+    if (Part != 0)
+      return State.get(this, 0, /*IsScalar*/ true);
+    Value *IncomingFromVPlanPred =
+        State.get(getOperand(0), Part, /* IsScalar */ true);
+    Value *IncomingFromOtherPred =
+        State.get(getOperand(1), Part, /* IsScalar */ true);
+    auto *NewPhi = Builder.CreatePHI(IncomingFromOtherPred->getType(), 2, Name);
+    BasicBlock *VPlanPred =
+        State.CFG
+            .VPBB2IRBB[cast<VPBasicBlock>(getParent()->getSinglePredecessor())];
+    NewPhi->addIncoming(IncomingFromVPlanPred, VPlanPred);
+    for (auto *BB : predecessors(Builder.GetInsertBlock()))
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ayalz wrote:

These `predecessors` exclude VPlanPred, namely middle block?

https://github.com/llvm/llvm-project/pull/94760