message: parser: give error when RHS is not const
diff: @@ -449,6 +449,9 @@ static unique_ptr<Instr> parse_binop(string_view name, token op_token) {
  case EXTRACTVALUE:
  op = BinOp::ExtractValue;
  rettype = &get_sym_type();
  +
  + if (!dynamic_cast<IntConst*>(&b))
  + error("Only int const accepted for RHS of extractvalue");
  break;
  default:
  UNREACHABLE();
  