pgsql: Add some defenses against constant-FALSE outer join conditions.

Log Message:
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Add some defenses against constant-FALSE outer join conditions.  Since
eval_const_expressions will generally throw away anything that's ANDed with
constant FALSE, what we're left with given an example like

select * from tenk1 a where (unique1,0) in (select unique2,1 from tenk1 b);

is a cartesian product computation, which is really not acceptable.
This is a regression in CVS HEAD compared to previous releases, which were
able to notice the impossible join condition in this case --- though not in
some related cases that are also improved by this patch, such as

select * from tenk1 a left join tenk1 b on (a.unique1=b.unique2 and 0=1);

Fix by skipping evaluation of the appropriate side of the outer join in
cases where it's demonstrably unnecessary.

Modified Files:
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    pgsql/src/backend/optimizer/path:
        joinrels.c (r1.93 -> r1.94)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/joinrels.c?r1=1.93&r2=1.94)