Fix a bug in 8.2.x that was exposed while investigating Kevin Grittner's

report of poor planning in 8.3: it's unsafe to push a constant across an
outer join when the outer-join condition is delayed by lower outer joins,
unless we recheck the outer-join condition at the upper outer join.
8.2.x doesn't really have the ability to tell whether this is the case
or not, but fortunately it doesn't matter --- it seems most desirable to
keep the join condition whether it's entirely redundant or not.  However,
it's usually mostly redundant, so force its selectivity to 1.0.

It might be a good idea to back-patch this into 8.1 as well, but I'll
refrain until/unless there's evidence that 8.1 actually fails on any
cases that this would fix.

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