Fix table group join issue with subclasses

[bahusoid]

Problematic scenario: invalid SQL is generated when joined entity with subclasses doesn't need subclass joins

//inner joined animal is not used in output (no need to join subclasses)
var resultsFromOuter1 = db.Animals.Join(db.Animals, o => o.Id, i => i.Id, (o, i) => o).Take(1).ToList();