[llvm] [VPlan] Add exit phi operands during initial construction (NFC). (PR #136455)

[via llvm-commits]

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@@ -1139,7 +1139,7 @@ InstructionCost VPIRInstruction::computeCost(ElementCount VF,
 void VPIRInstruction::extractLastLaneOfOperand(VPBuilder &Builder) {
   assert(isa<PHINode>(getInstruction()) &&
          "can only add exiting operands to phi nodes");
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ayalz wrote:

Independent: 
```suggestion
         "can only update exiting operands to phi nodes");
```

https://github.com/llvm/llvm-project/pull/136455