The InformixDialect also implements the getLimitHandler:

@Override
public LimitHandler getLimitHandler() {
	return FirstLimitHandler.INSTANCE;
}

The FirstLimitHandler.INSTANCE referenced there is also used in IngresDialect and TimesTenDialect. Not sure for those 2 but at least Informix 12 can now handle offsets.
And this FirstLimitHandler has more or less the same check in its processSql method.