pgsql: Fix longstanding bug that would sometimes let the planner

Log Message:
-----------
Fix longstanding bug that would sometimes let the planner generate a bad plan
for an outer join; symptom is bogus error "RIGHT JOIN is only supported with
merge-joinable join conditions".  Problem was that select_mergejoin_clauses
did its tests in the wrong order.  We need to force left join not right join
for a merge join when there are non-mergeable join clauses; but the test for
this only accounted for mergejoinability of the clause operator, and not
whether the left and right Vars were of the proper relations.  Per report
from Jean-Pierre Pelletier.

Modified Files:
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    pgsql/src/backend/optimizer/path:
        joinpath.c (r1.96 -> r1.97)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/joinpath.c.diff?r1=1.96&r2=1.97)