pgsql: Get rid of the rather fuzzily defined FlattenedSubLink node type

Log Message:
-----------
Get rid of the rather fuzzily defined FlattenedSubLink node type in favor of
making pull_up_sublinks() construct a full-blown JoinExpr tree representation
of IN/EXISTS SubLinks that it is able to convert to semi or anti joins.
This makes pull_up_sublinks() a shade more complex, but the gain in semantic
clarity is worth it.  I still have more to do in this area to address the
previously-discussed problems, but this commit in itself fixes at least one
bug in HEAD, as shown by added regression test case.

Modified Files:
--------------
    pgsql/src/backend/nodes:
        copyfuncs.c (r1.424 -> r1.425)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/copyfuncs.c?r1=1.424&r2=1.425)
        equalfuncs.c (r1.348 -> r1.349)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/equalfuncs.c?r1=1.348&r2=1.349)
        nodeFuncs.c (r1.37 -> r1.38)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/nodeFuncs.c?r1=1.37&r2=1.38)
        outfuncs.c (r1.352 -> r1.353)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/outfuncs.c?r1=1.352&r2=1.353)
    pgsql/src/backend/optimizer/plan:
        initsplan.c (r1.147 -> r1.148)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/plan/initsplan.c?r1=1.147&r2=1.148)
        subselect.c (r1.145 -> r1.146)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/plan/subselect.c?r1=1.145&r2=1.146)
    pgsql/src/backend/optimizer/prep:
        prepjointree.c (r1.62 -> r1.63)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/prep/prepjointree.c?r1=1.62&r2=1.63)
        prepunion.c (r1.165 -> r1.166)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/prep/prepunion.c?r1=1.165&r2=1.166)
    pgsql/src/backend/optimizer/util:
        clauses.c (r1.275 -> r1.276)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/util/clauses.c?r1=1.275&r2=1.276)
        var.c (r1.83 -> r1.84)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/util/var.c?r1=1.83&r2=1.84)
    pgsql/src/backend/rewrite:
        rewriteManip.c (r1.120 -> r1.121)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/rewrite/rewriteManip.c?r1=1.120&r2=1.121)
    pgsql/src/include/nodes:
        nodes.h (r1.220 -> r1.221)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/nodes.h?r1=1.220&r2=1.221)
        primnodes.h (r1.145 -> r1.146)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/primnodes.h?r1=1.145&r2=1.146)
        relation.h (r1.168 -> r1.169)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/relation.h?r1=1.168&r2=1.169)
    pgsql/src/include/optimizer:
        subselect.h (r1.35 -> r1.36)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/optimizer/subselect.h?r1=1.35&r2=1.36)
    pgsql/src/test/regress/expected:
        join.out (r1.35 -> r1.36)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/join.out?r1=1.35&r2=1.36)
        join_1.out (r1.16 -> r1.17)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/join_1.out?r1=1.16&r2=1.17)
    pgsql/src/test/regress/sql:
        join.sql (r1.26 -> r1.27)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/join.sql?r1=1.26&r2=1.27)