[RISCV] Check for register where immediate should be in RISCVInstrInfo::verifyInstruction. (#120286)

The generic verifier will do this if the operand type is
OPERAND_IMMEDIATE, but we use our own custom operand types. Immediate
operands are still allowed to be globals, constant pools, blockaddress,
etc. so we can't check !isImm().

Author: topperc

llvm/lib/Target/RISCV/RISCVInstrInfo.cpp

@@ -2451,6 +2451,10 @@ bool RISCVInstrInfo::verifyInstruction(const MachineInstr &MI,
     if (OpType >= RISCVOp::OPERAND_FIRST_RISCV_IMM &&
         OpType <= RISCVOp::OPERAND_LAST_RISCV_IMM) {
       const MachineOperand &MO = MI.getOperand(Index);
+      if (MO.isReg()) {
+        ErrInfo = "Expected a non-register operand.";
+        return false;
+      }
       if (MO.isImm()) {
         int64_t Imm = MO.getImm();
         bool Ok;