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/common/simpleAssignment.kt

@@ -0,0 +1,9 @@
+fun test() {
+    val x: /*T0@*/Int = 10/*LIT*/
+    val y: /*T1@*/Int = x/*T0@Int*/
+    val z: /*T2@*/Int = y/*T1@Int*/
+}
+
+//LOWER <: T0 due to 'INITIALIZER'
+//T0 <: T1 due to 'INITIALIZER'
+//T1 <: T2 due to 'INITIALIZER'