pgsql: Add code to eval_const_expressions() to support

Log Message:
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Add code to eval_const_expressions() to support const-simplification of
CoerceViaIO nodes.  This improves the ability of the planner to deal with
cases where the node input is a constant.  Per bug #4170.

Modified Files:
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    pgsql/src/backend/optimizer/util:
        clauses.c (r1.258 -> r1.259)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/util/clauses.c?r1=1.258&r2=1.259)