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

[via llvm-commits]

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@@ -864,6 +861,11 @@ void VPIRInstruction::execute(VPTransformState &State) {
   State.Builder.SetInsertPoint(I.getParent(), std::next(I.getIterator()));
 }
 
+InstructionCost VPIRInstruction::computeCost(ElementCount VF,
+                                             VPCostContext &Ctx) const {
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ayalz wrote:

Some comment explaining that this wraps an existing IR instruction on the border of VPlan's scope, hence does not contribute to VPlan's cost modeling.  

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