pgsql: Fix another semijoin-ordering bug.

Log Message:
-----------
Fix another semijoin-ordering bug. We already knew that we couldn't
reorder a semijoin into or out of the righthand side of another semijoin,
but actually it doesn't work to reorder it into or out of the righthand
side of a left or antijoin, either. Per bug #4906 from Mathieu Fenniak.

This was sloppy thinking on my part. This identity does work:

( A left join B on (Pab) ) semijoin C on (Pac)
==
( A semijoin C on (Pac) ) left join B on (Pab)

but I failed to see that that doesn't mean this does:

( A left join B on (Pab) ) semijoin C on (Pbc)
!=
A left join ( B semijoin C on (Pbc) ) on (Pab)

Modified Files:
--------------
pgsql/src/backend/optimizer:
README (r1.49 -> r1.50)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/README?r1=1.49&r2=1.50)
pgsql/src/backend/optimizer/plan:
initsplan.c (r1.154 -> r1.155)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/plan/initsplan.c?r1=1.154&r2=1.155)