[llvm] [VPlan] Sink retrieving legacy costs to more specific computeCost impls. (PR #109708)

[via llvm-commits]

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@@ -915,6 +915,9 @@ class VPSingleDefRecipe : public VPRecipeBase, public VPValue {
     llvm_unreachable("Unhandled VPDefID");
   }
 
+  InstructionCost computeCost(ElementCount VF,
+                              VPCostContext &Ctx) const override;
+
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ayalz wrote:

nit: can a better position be found, rather than between two static `classof()`'s?

https://github.com/llvm/llvm-project/pull/109708