[ConstantRange] Add exact union/intersect (NFC)

For some optimizations on comparisons it's necessary that the
union/intersect is exact and not a superset. Add methods that
return Optional<ConstantRange> only if the result is exact.

For the sake of simplicity this is implemented by comparing
the subset and superset approximations for now, but it should be
possible to do this more directly, as unionWith() and intersectWith()
already distinguish the cases where the result is imprecise for the
preferred range type functionality.