Simplify positive intersection co-occurrences too

[LPTK]

The following is currently not simplified:

class Some[A]
//│ Defined class Some

def f: (Some['A] & 'this) -> (Some['A] & 'this)
//│ f: (Some['A] & 'this) -> (Some['A] & 'this)
//│  = <missing implementation>

But we could simplify it to (Some['A] & 'this) -> 'this because 'this always co-occurs with Some['A] negatively.

We could also probably do the same for unions, though it would be less straightforward to implement since we put types in DNF, not CNF.