pgsql: Skip eval_const_expressions when the query is such that the

Log Message:
-----------
Skip eval_const_expressions when the query is such that the expression
would be evaluated only once anyway (ie, it's just a SELECT with no
FROM or an INSERT ... VALUES).  The planner can't do it any faster than
the executor, so no point in an extra copying of the expression tree.

Modified Files:
--------------
    pgsql/src/backend/optimizer/plan:
        planner.c (r1.186 -> r1.187)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/plan/planner.c.diff?r1=1.186&r2=1.187)