pgsql: Teach const-expression simplification to simplify boolean

Log Message:
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Teach const-expression simplification to simplify boolean equality cases,
that is 'x = true' becomes 'x' and 'x = false' becomes 'NOT x'. This isn't
all that amazingly useful in itself, but it ensures that we will recognize
the different forms as being logically equivalent when checking partial
index predicates. Per example from Patrick Clery.

Modified Files:
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pgsql/src/backend/optimizer/util:
clauses.c (r1.188 -> r1.189)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/util/clauses.c.diff?r1=1.188&r2=1.189)