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/compareWithNull.kt

@@ -0,0 +1,19 @@
+fun a(): /*T0@*/Int? {
+    return 42/*LIT*/
+}
+
+val b: /*T1@*/Int? = 2/*LIT*/
+
+fun c(p: /*T2@*/Int?) {
+    if (p/*T2@Int*/ == null/*LIT*/);
+}
+
+fun check() {
+    if (a()/*T0@Int*/ == null/*LIT*/ || b/*T1@Int*/ == null/*LIT*//*LIT*/);
+}
+
+//LOWER <: T0 due to 'RETURN'
+//LOWER <: T1 due to 'INITIALIZER'
+//T2 := UPPER due to 'COMPARE_WITH_NULL'
+//T0 := UPPER due to 'COMPARE_WITH_NULL'
+//T1 := UPPER due to 'COMPARE_WITH_NULL'