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

[via llvm-commits]

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

nit: this should be used to get a **vector** Value.

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