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/nullAsAssignment.kt

@@ -0,0 +1,18 @@
+fun test() {
+    var x: /*T0@*/Int? = 1/*LIT*/
+    x/*T0@Int*/ = null/*NULL!!U*/
+
+    var y: /*T1@*/Int? = 5/*LIT*/
+    y/*T1@Int*/ = nullableFun()/*T2@Int*/
+}
+
+fun nullableFun(): /*T2@*/Int? {
+    return null/*NULL!!U*/
+}
+
+//LOWER <: T0 due to 'INITIALIZER'
+//UPPER <: T0 due to 'ASSIGNMENT'
+//T0 := UPPER due to 'COMPARE_WITH_NULL'
+//LOWER <: T1 due to 'INITIALIZER'
+//T2 <: T1 due to 'ASSIGNMENT'
+//UPPER <: T2 due to 'RETURN'