[llvm] [VPlan] Compute scalar cost based on VPlan0 instead of legacy CM. (PR #196845)

[David Sherwood via llvm-commits]

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@@ -1357,6 +1376,13 @@ InstructionCost VPRecipeWithIRFlags::getCostForRecipeWithOpcode(
 
 InstructionCost VPInstruction::computeCost(ElementCount VF,
                                            VPCostContext &Ctx) const {
+  // A scalar cost is only computed for VPlan0, which has no vector-only
+  // opcodes.
+  assert(!(VF.isScalar() &&
+           (isVectorToScalar() ||
+            getOpcode() == VPInstruction::FirstOrderRecurrenceSplice)) &&
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david-arm wrote:

I'm surprised we don't already have a utility function for this. Is there any value in creating a `isVectorOnly` utility routine? Not sure if it would be useful elsewhere though.

https://github.com/llvm/llvm-project/pull/196845