New J2K: separate nullability inference from common one & nullability bug fixes

It will be needed for structure mutability inference

#KT-21467 fixed
#KT-32609 fixed
#KT-32572 fixed
#KT-24677 fixed

nj2k/testData/newJ2k/overloads/Override.kt

@@ -21,13 +21,11 @@ internal open class A : I {
     }
 
     fun bar(i: Int) {}
-
     open fun x() {
         x(1)
     }
 
     fun x(i: Int) {}
-
     fun y() {
         y(1)
     }