[RISCV] Use getRISCVInstructionCost for split cost in mask reductions. NFC

This is effectively the same due to how the mask instructions have an
LMUL of 1 and cost of 1, but matches how we use LT.first elsewhere in
RISCVTargetTransformInfo.cpp by using it to multiply another
instruction cost.

Author: lukel97

llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp

@@ -1489,35 +1489,37 @@ RISCVTTIImpl::getArithmeticReductionCost(unsigned Opcode, VectorType *Ty,
     return BaseT::getArithmeticReductionCost(Opcode, Ty, FMF, CostKind);
 
   std::pair<InstructionCost, MVT> LT = getTypeLegalizationCost(Ty);
-  SmallVector<unsigned, 3> Opcodes;
   Type *ElementTy = Ty->getElementType();
   if (ElementTy->isIntegerTy(1)) {
     if (ISD == ISD::AND) {
       // Example sequences:
       //   vsetvli a0, zero, e8, mf8, ta, ma
+      //   vmand.mm v8, v9, v8 ; needed every time type is split
       //   vmnot.m v8, v0
       //   vcpop.m a0, v8
       //   seqz a0, a0
-      Opcodes = {RISCV::VMNAND_MM, RISCV::VCPOP_M};
-      return (LT.first - 1) +
-             getRISCVInstructionCost(Opcodes, LT.second, CostKind) +
+      return LT.first * getRISCVInstructionCost(RISCV::VMNAND_MM, LT.second,
+                                                CostKind) +
+             getRISCVInstructionCost(RISCV::VCPOP_M, LT.second, CostKind) +
              getCmpSelInstrCost(Instruction::ICmp, ElementTy, ElementTy,
                                 CmpInst::ICMP_EQ, CostKind);
     } else {
       // Example sequences:
       //   vsetvli a0, zero, e8, mf8, ta, ma
+      //   vmxor.mm v8, v9, v8 ; needed every time type is split
       //   vcpop.m a0, v0
       //   snez a0, a0
-      Opcodes = {RISCV::VCPOP_M};
-      return (LT.first - 1) +
-             getRISCVInstructionCost(Opcodes, LT.second, CostKind) +
+      return (LT.first - 1) *
+                 getRISCVInstructionCost(RISCV::VMXOR_MM, LT.second, CostKind) +
+             getRISCVInstructionCost(RISCV::VCPOP_M, LT.second, CostKind) +
              getCmpSelInstrCost(Instruction::ICmp, ElementTy, ElementTy,
                                 CmpInst::ICMP_NE, CostKind);
     }
   }
 
   // IR Reduction is composed by two vmv and one rvv reduction instruction.
   unsigned SplitOp;
+  SmallVector<unsigned, 3> Opcodes;
   switch (ISD) {
   case ISD::ADD:
     SplitOp = RISCV::VADD_VV;