[RISCV][TTI] Simplify code using getRealVLen() [NFC]

Author: preames

llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp

@@ -721,11 +721,9 @@ InstructionCost RISCVTTIImpl::getShuffleCost(TTI::ShuffleKind Kind,
     // TODO: Extend for scalable subvector types
     if (std::pair<InstructionCost, MVT> SubLT = getTypeLegalizationCost(SubTp);
         SubLT.second.isValid() && SubLT.second.isFixedLengthVector()) {
-      const unsigned MinVLen = ST->getRealMinVLen();
-      const unsigned MaxVLen = ST->getRealMaxVLen();
-      if (MinVLen == MaxVLen &&
-          SubLT.second.getScalarSizeInBits() * Index % MinVLen == 0 &&
-          SubLT.second.getSizeInBits() <= MinVLen)
+      if (std::optional<unsigned> VLen = ST->getRealVLen();
+          VLen && SubLT.second.getScalarSizeInBits() * Index % *VLen == 0 &&
+          SubLT.second.getSizeInBits() <= *VLen)
         return TTI::TCC_Free;
     }