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

[via llvm-commits]

================
@@ -593,13 +601,35 @@ Value *VPInstruction::generatePerPart(VPTransformState &State, unsigned Part) {
     Value *Addend = State.get(getOperand(1), Part, /* IsScalar */ true);
     return Builder.CreatePtrAdd(Ptr, Addend, Name);
   }
+  case VPInstruction::ResumePhi: {
+    if (Part != 0)
+      return State.get(this, 0, /*IsScalar*/ true);
+    Value *IncomingFromVPlanPred =
+        State.get(getOperand(0), Part, /* IsScalar */ true);
+    Value *IncomingFromOtherPreds =
+        State.get(getOperand(1), Part, /* IsScalar */ true);
+    auto *NewPhi =
+        Builder.CreatePHI(IncomingFromOtherPreds->getType(), 2, Name);
+    BasicBlock *VPlanPred =
+        State.CFG
+            .VPBB2IRBB[cast<VPBasicBlock>(getParent()->getSinglePredecessor())];
+    NewPhi->addIncoming(IncomingFromVPlanPred, VPlanPred);
+    for (auto *OtherPred : predecessors(Builder.GetInsertBlock())) {
+      assert(OtherPred != VPlanPred &&
+             "VPlan predecessors should not be connected yet");
+      NewPhi->addIncoming(IncomingFromOtherPreds, OtherPred);
+    }
----------------
ayalz wrote:

```suggestion
    for (auto *OtherPred : predecessors(Builder.GetInsertBlock())) {
      assert(OtherPred != VPlanPred &&
             "VPlan predecessors should not be connected yet");
      NewPhi->addIncoming(IncomingFromOtherPreds, OtherPred);
    }
    NewPhi->addIncoming(IncomingFromVPlanPred, VPlanPred);
```
a bit more logical to first connect all existing predecessors asserting to not be VPlanPred, and then connect the latter(?)

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