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

[via llvm-commits]

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@@ -883,8 +888,11 @@ void VPlan::execute(VPTransformState *State) {
 
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ayalz wrote:

```suggestion
  bool IsScalarNeeded = isa<VPCanonicalIVPHIRecipe>(PhiR);
```



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