Fix Atomic#max and #min for signed enums

[HertzDevil]

Since enum types are not subtypes of their base types, the check T < Int::Signed in Atomic(T)#max and #min will always return false for enum types, which makes signed enums always use the unsigned interpretation. This PR adds the necessary runtime type checks. (The TypeNode#base_type suggested in #9803 could turn this runtime check into a compile-time one.)

[beta-ziliani]

Definitively a good example for base_type macro method. In any case, better be correct first, and at a later stage we can rewrite it to avoid the runtime cost.