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

[via llvm-commits]

================
@@ -407,7 +408,7 @@ Value *VPInstruction::generateInstruction(VPTransformState &State,
   }
   case VPInstruction::ComputeReductionResult: {
     if (Part != 0)
-      return State.get(this, 0);
+      return State.get(this, VPIteration(0, 0));
----------------
ayalz wrote:

```suggestion
      return State.get(this, 0, true /* scalar */);
```

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