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/inference/nullability/superMethod.kt

@@ -0,0 +1,18 @@
+open class A () {
+    open fun foo(x: /*T0@*/Int?): /*T1@*/Int? {
+        if (x/*T0@Int*/ == null/*LIT*/);
+        return null/*NULL!!U*/
+    }
+}
+
+class B : A() {
+    override fun foo(x: /*T2@*/Int?): /*T3@*/Int? {
+        return 1/*LIT*/
+    }
+}
+
+//T0 := UPPER due to 'COMPARE_WITH_NULL'
+//UPPER <: T1 due to 'RETURN'
+//LOWER <: T3 due to 'RETURN'
+//T1 := T3 due to 'SUPER_DECLARATION'
+//T0 := T2 due to 'SUPER_DECLARATION'