Fix SELECT DISTINCT with index-optimized MIN/MAX on inheritance trees.

Fix SELECT DISTINCT with index-optimized MIN/MAX on inheritance trees.

In a query such as "SELECT DISTINCT min(x) FROM tab", the DISTINCT is
pretty useless (there being only one output row), but nonetheless it
shouldn't fail.  But it could fail if "tab" is an inheritance parent,
because planagg.c's code for fixing up equivalence classes after making the
index-optimized MIN/MAX transformation wasn't prepared to find child-table
versions of the aggregate expression.  The least ugly fix seems to be
to add an option to mutate_eclass_expressions() to skip child-table
equivalence class members, which aren't used anymore at this stage of
planning so it's not really necessary to fix them.  Since child members
are ignored in many cases already, it seems plausible for
mutate_eclass_expressions() to have an option to ignore them too.

Per bug #7703 from Maxim Boguk.

Back-patch to 9.1.  Although the same code exists before that, it cannot
encounter child-table aggregates AFAICS, because the index optimization
transformation cannot succeed on inheritance trees before 9.1 (for lack
of MergeAppend).