Re: Evaluate expression at planning time for two more cases

I'm not sure what I think about Ashutosh's ideas about doing this
somewhere else than eval_const_expressions.  I do not buy the argument
that it's interesting to do this separately for each child partition.
Child partitions that have attnotnull constraints different from their
parent's are at best a tiny minority use-case, if indeed we allow them
at all (I tend to think we shouldn't). 

As a not incidental example, consider

        select ... from t1 left join t2 on (...) where t2.x is not null;

reduce_outer_joins will realize that the left join can be reduced
to a plain join, whereupon (if t2.x is attnotnull) the WHERE clause
really is constant-true --- and this seems like a poster-child case
for it being useful to optimize away the WHERE clause.  But
we won't be able to detect that if we apply the optimization during
eval_const_expressions.  So maybe that's a good reason to do it
somewhere later.