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

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.