psi2ir: Fix implicit casts generation

Implicit cast from T to S is not required if
  T <: S.makeNullable()

compiler/testData/ir/irText/classes/dataClassesGeneric.kt

@@ -1 +1,5 @@
-data class Test1<T>(val x: T)
+data class Test1<T>(val x: T)
+
+data class Test2<T : Number>(val x: T)
+
+data class Test3<T>(val x: List<T>)