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

[via llvm-commits]

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@@ -1389,6 +1389,13 @@ class VPScalarCastRecipe : public VPSingleDefRecipe {
 
   /// Returns the result type of the cast.
   Type *getResultType() const { return ResultTy; }
+
+  bool onlyFirstLaneUsed(const VPValue *Op) const override {
+    // At the moment, only scalar codegen is implemented.
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ayalz wrote:

```suggestion
    // At the moment, only uniform codegen is implemented.
```
?
scalar codegen in general may use other lanes.

Can this improvement of onlyFirstLaneUsed() be pushed independently?

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