Previously, with the `index_experimental` feature enabled, the query in
the added test would enter an infinite loop. This happened because
`label_grouping_agg_step` pointed to a constant argument that was moved
to the end of the program. As a result, the aggregation loop would jump
to the constant, then return to the start of the main loop, rewind the
index, and re-enter the aggregation loop—causing it to repeat
indefinitely.
Previously, queries like:
```
SELECT
CASE WHEN c0 != 'x' THEN group_concat(c1, ',') ELSE 'x' END
FROM t0
GROUP BY c0;
```
would return incorrect results because c0 was not copied during the
aggregation loop into a register accessible to the logic processing the
grouped results (e.g., the CASE WHEN expression in this example).
The same issue applied to expressions in the HAVING and ORDER BY clauses.
Previously, the logic for collecting non-aggregate columns was duplicated
across multiple locations and implemented inconsistently. This caused a
bug that was revealed by the refactoring in this commit (see the added
test).
