[RISCV] Fix -Wunused-variable in RISCVISelLowering.cpp (NFC)

/llvm-project/llvm/lib/Target/RISCV/RISCVISelLowering.cpp:21558:14: error: unused variable 'ValLMUL' [-Werror,-Wunused-variable]
    unsigned ValLMUL =
             ^
/llvm-project/llvm/lib/Target/RISCV/RISCVISelLowering.cpp:21561:14: error: unused variable 'PartLMUL' [-Werror,-Wunused-variable]
    unsigned PartLMUL =
             ^
2 errors generated.

Author: DamonFool

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -21555,10 +21555,10 @@ bool RISCVTargetLowering::splitValueIntoRegisterParts(
 
   if (ValueVT.isRISCVVectorTuple() && PartVT.isRISCVVectorTuple()) {
     unsigned ValNF = ValueVT.getRISCVVectorTupleNumFields();
-    unsigned ValLMUL =
+    [[maybe_unused]] unsigned ValLMUL =
         divideCeil(ValueVT.getSizeInBits(), ValNF * RISCV::RVVBitsPerBlock);
     unsigned PartNF = PartVT.getRISCVVectorTupleNumFields();
-    unsigned PartLMUL =
+    [[maybe_unused]] unsigned PartLMUL =
         divideCeil(PartVT.getSizeInBits(), PartNF * RISCV::RVVBitsPerBlock);
     assert(ValNF == PartNF && ValLMUL == PartLMUL &&
            "RISC-V vector tuple type only accepts same register class type "