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.
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