[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 uniform codegen is implemented.
+    assert(is_contained(operands(), Op) &&
+           "Op must be an operand of the recipe");
+    return true;
+  }
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ayalz wrote:

Can be pushed separately but only testable with rest of patch?

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