pgsql: Second try at making examine_variable and friends behave sanely

Log Message:
-----------
Second try at making examine_variable and friends behave sanely in
cases with binary-compatible relabeling.  My first try was implicitly
assuming that all operators scalarineqsel is used for have binary-
compatible datatypes on both sides ... which is very wrong of course.
Per report from Michael Fuhr.

Modified Files:
--------------
    pgsql/src/backend/utils/adt:
        selfuncs.c (r1.175 -> r1.176)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/selfuncs.c.diff?r1=1.175&r2=1.176)