[gen4 planner] Make sure to not push down expressions when not possible

[frouioui]

Description

This PR fixes #12540 by making sure to not push down to MySQL complex expressions that involved aggregations.

If we have a derived table and it's aggregating results for us, we can't use these results until the are fully aggregated, which happens on the vtgate level.

select dt.a,
       dt.b,
       (dt.a / dt.b)
from (select sum(a) as a,
             sum(b) as b
      from user) dt

In the query above, since we have to spread out the aggregation across all user shards, we can't push down the division between the two sums until after summing up the individual shard results.

Related Issue(s)

Fixes #12540

Checklist

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[systay]

I don't this is correct. The query we are trying to fix should not be using the AggregateRandom expression here in the first place

[frouioui]

LGTM

[arthurschreiber]

❤️