Checks if anonymous function can be converted to eta expansion with or without placeholder.
Also checks if placeholder can be removed from eta expansion.
For example
def f(x: Any, y: Int) {}
val x = f(_, _)
val y: (Int, Int) => Unit = f _
can be rewritten as
def f(x: Any, y: Int) {}
val x = A.f _
val y: (Int, Int) => Unit = A.f