[llvm] [VPlan] Factor out logic to common compute costs to helper (NFCI). (PR #153361)

[David Sherwood via llvm-commits]

================
@@ -940,28 +940,88 @@ Value *VPInstruction::generate(VPTransformState &State) {
   }
 }
 
+std::optional<InstructionCost>
+VPRecipeWithIRFlags::getCommonCost(unsigned Opcode, ElementCount VF,
+                                   VPCostContext &Ctx, bool IsVector) const {
+  Type *ScalarTy = Ctx.Types.inferScalarType(this);
+  Type *ResultTy = IsVector ? toVectorTy(ScalarTy, VF) : ScalarTy;
+  switch (Opcode) {
+  case Instruction::FNeg:
+    return Ctx.TTI.getArithmeticInstrCost(
+        Opcode, ResultTy, Ctx.CostKind,
+        {TargetTransformInfo::OK_AnyValue, TargetTransformInfo::OP_None},
+        {TargetTransformInfo::OK_AnyValue, TargetTransformInfo::OP_None});
----------------
david-arm wrote:

Can't you just do

```
    return Ctx.TTI.getArithmeticInstrCost(Opcode, ResultTy, Ctx.CostKind);
```

given that it already has the default parameters that you want?

```
  LLVM_ABI InstructionCost getArithmeticInstrCost(
      unsigned Opcode, Type *Ty,
      TTI::TargetCostKind CostKind = TTI::TCK_RecipThroughput,
      TTI::OperandValueInfo Opd1Info = {TTI::OK_AnyValue, TTI::OP_None},
      TTI::OperandValueInfo Opd2Info = {TTI::OK_AnyValue, TTI::OP_None},
      ArrayRef<const Value *> Args = {}, const Instruction *CxtI = nullptr,
      const TargetLibraryInfo *TLibInfo = nullptr) const;
```

https://github.com/llvm/llvm-project/pull/153361