[VPlan] Move FOR splice cost into VPInstruction::FirstOrderRecurrenceSplice

After #124093 we now support fixed-order recurrences with EVL tail folding by replacing VPInstruction::FirstOrderRecurrenceSplice with a VP splice intrinsic.

However the costing for the splice is currently done in VPFirstOrderRecurrencePHIRecipe, so when we add the VP splice intrinsic we end up costing it twice.

This fixes it by splitting out the cost for the splice into FirstOrderRecurrenceSplice so that it's not duplicated when we replace it.

We still have to keep the VF=1 checks in VPFirstOrderRecurrencePHIRecipe since the splice might end up dead and discarded, e.g. in the test @pr97452_scalable_vf1_for.

Author: lukel97

llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp

@@ -743,6 +743,17 @@ InstructionCost VPInstruction::computeCost(ElementCount VF,
     return Ctx.TTI.getArithmeticReductionCost(
         Instruction::Or, cast<VectorType>(VecTy), std::nullopt, Ctx.CostKind);
   }
+  case VPInstruction::FirstOrderRecurrenceSplice: {
+    assert(VF.isVector());
+    SmallVector<int> Mask(VF.getKnownMinValue());
+    std::iota(Mask.begin(), Mask.end(), VF.getKnownMinValue() - 1);
+    Type *VectorTy =
+        toVectorTy(Ctx.Types.inferScalarType(this->getVPSingleValue()), VF);
+
+    return Ctx.TTI.getShuffleCost(TargetTransformInfo::SK_Splice,
+                                  cast<VectorType>(VectorTy), Mask,
+                                  Ctx.CostKind, VF.getKnownMinValue() - 1);
+  }
   default:
     // TODO: Compute cost other VPInstructions once the legacy cost model has
     // been retired.
@@ -3463,14 +3474,7 @@ VPFirstOrderRecurrencePHIRecipe::computeCost(ElementCount VF,
   if (VF.isScalable() && VF.getKnownMinValue() == 1)
     return InstructionCost::getInvalid();
 
-  SmallVector<int> Mask(VF.getKnownMinValue());
-  std::iota(Mask.begin(), Mask.end(), VF.getKnownMinValue() - 1);
-  Type *VectorTy =
-      toVectorTy(Ctx.Types.inferScalarType(this->getVPSingleValue()), VF);
-
-  return Ctx.TTI.getShuffleCost(TargetTransformInfo::SK_Splice,
-                                cast<VectorType>(VectorTy), Mask, Ctx.CostKind,
-                                VF.getKnownMinValue() - 1);
+  return 0;
 }
 
 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)

llvm/test/Transforms/LoopVectorize/RISCV/vplan-vp-intrinsics-fixed-order-recurrence.ll

@@ -51,7 +51,7 @@ define void @first_order_recurrence(ptr noalias %A, ptr noalias %B, i64 %TC) {
 ; IF-EVL-NEXT:   EMIT vp<[[RESUME_EXTRACT:%.+]]> = extract-from-end ir<[[LD]]>, ir<1>
 ; IF-EVL-NEXT:   EMIT branch-on-cond ir<true>
 ; IF-EVL-NEXT: Successor(s): ir-bb<for.end>, scalar.ph
-; IF-EVL: Cost of 4 for VF vscale x 4: FIRST-ORDER-RECURRENCE-PHI ir<[[FOR_PHI]]> = phi ir<33>, ir<[[LD]]>
+; IF-EVL: Cost of 0 for VF vscale x 4: FIRST-ORDER-RECURRENCE-PHI ir<[[FOR_PHI]]> = phi ir<33>, ir<[[LD]]>
 ; IF-EVL: Cost of 4 for VF vscale x 4: WIDEN-INTRINSIC vp<[[SPLICE]]> = call llvm.experimental.vp.splice(ir<[[FOR_PHI]]>, ir<[[LD]]>, ir<-1>, ir<true>, vp<[[PREV_EVL]]>, vp<[[EVL]]>)
 entry:
   br label %for.body