pgsql: Fix a performance regression in 8.2: optimization of MIN/MAX into

Log Message:
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Fix a performance regression in 8.2: optimization of MIN/MAX into indexscans
had stopped working for tables buried inside views or sub-selects.  This is
because I had gotten rid of the simplify_jointree() preprocessing step, and
optimize_minmax_aggregates() wasn't smart enough to deal with a non-canonical
FromExpr.  Per gripe from Bill Howe.

Modified Files:
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    pgsql/src/backend/optimizer/plan:
        planagg.c (r1.25 -> r1.26)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/plan/planagg.c.diff?r1=1.25&r2=1.26)