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

[David Sherwood via llvm-commits]

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@@ -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;
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david-arm wrote:

I think it's worth adding a comment explaining the discrepancy between `IsVector` and VF as it could be confusing.

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