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

[via llvm-commits]

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@@ -9289,7 +9288,7 @@ void VPReductionRecipe::execute(VPTransformState &State) {
     else
       NextInChain = State.Builder.CreateBinOp(
           (Instruction::BinaryOps)RdxDesc.getOpcode(Kind), NewRed, PrevInChain);
-    State.set(this, NextInChain, Part);
+    State.set(this, NextInChain, VPIteration(Part, 0));
----------------
ayalz wrote:

```suggestion
    State.set(this, NextInChain, Part, true /* scalar */);
```
same API of `get` should apply to `set` as well.

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