pgsql: When adding a "target IS NOT NULL" indexqual to the plan for an

Log Message:
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When adding a "target IS NOT NULL" indexqual to the plan for an index-optimized
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.

Modified Files:
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    pgsql/src/backend/optimizer/plan:
        planagg.c (r1.51 -> r1.52)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/plan/planagg.c?r1=1.51&r2=1.52)