[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.
   Value *get(VPValue *Def, unsigned Part);
 
+  /// Get the generated Value for a given VPValue \p Def and a given \p Part or
+  /// lane 0 for the part if \p NeedsScalar is true. \see set.
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fhahn wrote:

Updated and renamed the arg to `IsScalar` to hopefully clarify.

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