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/smartCast.kt

@@ -0,0 +1,7 @@
+fun foo(o: /*T0@*/Int??) {
+    if (o/*T0@Int*/ == null/*LIT*/) return
+    val a: /*T1@*/Int = o/*T0@Int!!L*/
+}
+
+//T0 := UPPER due to 'COMPARE_WITH_NULL'
+//LOWER <: T1 due to 'INITIALIZER'