An available index is never used for ordering, when the requested order is DESC

```
CREATE TABLE my_table(K1 INT);
CREATE INDEX my_index ON my_table(K1);
EXPLAIN PLAN FOR SELECT * FROM my_table ORDER BY K1 DESC;
```

This *should* lead to an efficient ordering via the given index. However, the explanation does not contain `/* index sorted */`, i.e. the index is not used. However, the index is used for `ASC`.

See also #699.