[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");
-  assert(getNumOperands() == 1 && "must have a single operand");
+  assert(getNumOperands() > 0 && "must have at least one operand");
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ayalz wrote:

And yet we continue to extract the first operand, only? extractLastLaneOf[First]Operand()

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