pgsql: Tweak make_inh_translation_lists() to check the common case

Log Message:
-----------
Tweak make_inh_translation_lists() to check the common case wherein parent and
child attnums are the same, before it grovels through each and every child
column looking for a name match.  Saves some time in large inheritance trees,
per example from Greg.

Modified Files:
--------------
    pgsql/src/backend/optimizer/prep:
        prepunion.c (r1.140 -> r1.141)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/prep/prepunion.c.diff?r1=1.140&r2=1.141)