Code review for match_clause_to_partition_key().

Fix inconsistent decisions about NOMATCH vs UNSUPPORTED result codes.
If we're going to cater for partkeys that have the same expression and
different collations, surely we should also support partkeys with the
same expression and different opclasses.

Clean up shaky handling of commuted opclauses, eg checking the wrong
operator to see what its negator is.  This wouldn't cause any actual
bugs given a sane opclass definition, but it doesn't seem helpful to
expend more code to be less correct.

Improve handling of null elements in ScalarArrayOp arrays: in the
"op ALL" case, we can conclude they result in an unsatisfiable clause.

Minor cosmetic changes and comment improvements.