[RISCV] Replace a hardcoded 16 with RISCVZC::INVALID_RLIST. NFC

Author: topperc

llvm/lib/Target/RISCV/AsmParser/RISCVAsmParser.cpp

@@ -2576,7 +2576,7 @@ ParseStatus RISCVAsmParser::parseReglist(OperandVector &Operands) {
     RegEnd = RegStart;
 
   auto Encode = RISCVZC::encodeRlist(RegEnd, IsEABI);
-  if (Encode == 16)
+  if (Encode == RISCVZC::INVALID_RLIST)
     return Error(S, "invalid register list");
   Operands.push_back(RISCVOperand::createRlist(Encode, S));