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/loopIterator.kt

@@ -0,0 +1,8 @@
+fun test(a: /*T1@*/List</*T0@*/Int>) {
+    for (i: /*T2@*/Int in a/*T1@List<T0@Int>*/) {
+
+    }
+}
+
+//T2 <: T0 due to 'ASSIGNMENT'
+//T1 := LOWER due to 'USE_AS_RECEIVER'