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

[via llvm-commits]

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@@ -334,6 +334,13 @@ Value *VPTransformState::get(VPValue *Def, unsigned Part) {
   return VectorValue;
 }
 
+Value *VPTransformState::get(VPValue *Def, unsigned Part,
+                             unsigned NeedsScalar) {
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ayalz wrote:

There's a slight redundancy when a vector is needed - either pass `false` or call get(Def, Part).
Perhaps worth extending get(Def, Part) to accept an optional `bool NeedsScalar = false`, and have it redirect to return get(Def, VPIteration(Part, 0))?

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