[RISCV] Fix a warning

This patch fixes:

  llvm/lib/Target/RISCV/RISCVISelLowering.cpp:8411:7: error: unused
  variable 'ArgVT' [-Werror,-Wunused-variable]

Author: kazutakahirata

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -8411,6 +8411,7 @@ SDValue RISCVTargetLowering::lowerPARTIAL_REDUCE_MLA(SDValue Op,
   MVT ArgVT = A.getSimpleValueType();
   assert(ArgVT == B.getSimpleValueType() &&
          ArgVT.getVectorElementType() == MVT::i8);
+  (void)ArgVT;
 
   // The zvqdotq pseudos are defined with sources and destination both
   // being i32.  This cast is needed for correctness to avoid incorrect