pgsql: Fix an oversight in join-removal optimization: we have to check

Log Message:
-----------
Fix an oversight in join-removal optimization: we have to check not only for
plain Vars that are generated in the inner rel and used above the join, but
also for PlaceHolderVars.  Per report from Oleg K.

Modified Files:
--------------
    pgsql/src/backend/optimizer/path:
        joinpath.c (r1.130 -> r1.131)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/joinpath.c?r1=1.130&r2=1.131)
    pgsql/src/test/regress/expected:
        join.out (r1.43 -> r1.44)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/join.out?r1=1.43&r2=1.44)
    pgsql/src/test/regress/sql:
        join.sql (r1.32 -> r1.33)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/join.sql?r1=1.32&r2=1.33)