[llvm] [VPlan] Consistently use (Part, 0) for first lane scalar values (PR #80271)

[via llvm-commits]

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@@ -9243,7 +9243,7 @@ void VPInterleaveRecipe::execute(VPTransformState &State) {
 
 void VPReductionRecipe::execute(VPTransformState &State) {
   assert(!State.Instance && "Reduction being replicated.");
-  Value *PrevInChain = State.get(getChainOp(), 0);
+  Value *PrevInChain = State.get(getChainOp(), VPIteration(0, 0));
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ayalz wrote:

```suggestion
  Value *PrevInChain = State.get(getChainOp(), 0, true /* scalar */);
```

https://github.com/llvm/llvm-project/pull/80271