[llvm] [AArch64][LV] Reduce cost of scaled reduction extends (PR #134074)

[David Sherwood via llvm-commits]

================
@@ -1796,6 +1796,11 @@ InstructionCost VPWidenCastRecipe::computeCost(ElementCount VF,
   // For Z/Sext, get the context from the operand.
   else if (Opcode == Instruction::ZExt || Opcode == Instruction::SExt ||
            Opcode == Instruction::FPExt) {
+    // If the extend is performed as part of another operation, it can be
+    // considered 'free'.
+    const VPlan *Plan = getParent()->getPlan();
+    if (Plan->isScaledReductionExtension(getUnderlyingInstr()))
+      return TargetTransformInfo::TCC_Free;
----------------
david-arm wrote:

In fact, can't you just look at uses of `this` recipe and if all uses are for the VPPartialReductionRecipe then mark as free? I just wonder if this avoids having to keep a pointer set.

https://github.com/llvm/llvm-project/pull/134074