[RISCV] Remove X26 from encodeRlist. NFC

The caller already checks X26 and generates its own error.

Author: topperc

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

@@ -2753,8 +2753,7 @@ ParseStatus RISCVAsmParser::parseReglist(OperandVector &Operands) {
     RegEnd = RegStart;
 
   auto Encode = RISCVZC::encodeRlist(RegEnd, IsEABI);
-  if (Encode == RISCVZC::INVALID_RLIST)
-    return Error(S, "invalid register list");
+  assert(Encode != RISCVZC::INVALID_RLIST);
   Operands.push_back(RISCVOperand::createRlist(Encode, S));
 
   return ParseStatus::Success;

llvm/lib/Target/RISCV/MCTargetDesc/RISCVBaseInfo.h

@@ -617,8 +617,6 @@ inline unsigned encodeRlist(MCRegister EndReg, bool IsRV32E = false) {
     return RLISTENCODE::RA_S0_S8;
   case RISCV::X25:
     return RLISTENCODE::RA_S0_S9;
-  case RISCV::X26:
-    return RLISTENCODE::INVALID_RLIST;
   case RISCV::X27:
     return RLISTENCODE::RA_S0_S11;
   default: