MIN or MAX, we must take care to insert the added qual in a legal place among
the existing indexquals, if any.  The btree index AM requires the quals to
appear in index-column order.  We didn't have to worry about this before
because "target IS NOT NULL" was just treated as a plain scan filter condition;
but as of 9.0 it can be an index qual and then it has to follow the rule.
Per report from Ian Barwick.