[9.x] Fix pure enums validation

[trckster]

There was a problem when passing UnitEnum instance to the Enum rule, not BackedEnum. UnitEnum doesn't have tryFrom method, but in the first condition there is a check that such method exists.
Basically, what we should do is just swap first condition with the second. If $value instanceof $this->type is true, it is surely enum, no matter Backed or not.