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

[Florian Hahn via llvm-commits]

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@@ -263,6 +263,10 @@ struct VPTransformState {
   /// \see set.
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fhahn wrote:

Folded into the variant that takes an optional `IsScalar` arg and clarified when a vector is returned.

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