be: assert that be_parse_arg() is only called before be is initialized

Without the assert it is easy to run into suble bugs because
be_get_backend_param() was called before all arguments are set by
be_parse_arg().

ir/be/bemain.c

@@ -227,6 +227,10 @@ int be_parse_arg(const char *arg)
 		lc_opt_print_help_for_entry(be_grp, '-', stdout);
 		return -1;
 	}
+
+	/* backend args may not have an effect anymore after the backend
+	 * has been initialized */
+	assert(!isa_initialized);
 	return lc_opt_from_single_arg(be_grp, arg);
 }