pgsql: My recent fix for semijoin planning didn't actually work for a

Log Message:
-----------
My recent fix for semijoin planning didn't actually work for a semijoin with a
RHS that can't be unique-ified --- join_is_legal has to check that before
deciding to build a join, else we'll have an unimplementable joinrel.
Per report from Greg Stark.

Modified Files:
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pgsql/src/backend/optimizer/path:
joinrels.c (r1.95 -> r1.96)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/joinrels.c?r1=1.95&r2=1.96)