Further improvements in cnfify: reduce amount of self-recursion

in or_normalize, remove detection of duplicate subexpressions (since it's
highly unlikely to be worth the amount of time it takes), and introduce
a dnfify() entry point so that unintelligible backwards logic in UNION
processing can be eliminated.  This is just an intermediate step ---
next thing is to look at not forcing the qual into CNF form when it would
be better off in DNF form.