Support IF statement in Transform function for post aggregation

This is for queries like:
```
SELECT
IF AVG(met1) == 0 THEN 0 ELSE SUM(met0) / AVG(met1)
FROM "auctions"
WHERE event_time = 1601510400000
LIMIT 1
```

No files or tests are named. Start by tracing how the Transform function handles post-aggregation expressions, then follow the query path for the demonstrated IF expression. Done means the example query is accepted and evaluates the zero and nonzero branches correctly, with coverage for both cases.

Written by the indexing model from the issue text.