[FIR-TEST] Add new testdata generated after changes in previous commit

compiler/testData/diagnostics/tests/smartCasts/varnotnull/postfixNotnullClassIncrement.fir.kt

@@ -0,0 +1,13 @@
+// !WITH_NEW_INFERENCE
+class MyClass
+
+operator fun MyClass.inc(): MyClass { return null!! }
+
+public fun box() : MyClass? {
+    var i : MyClass?
+    i = MyClass()
+    // type of j can be inferred as MyClass()
+    var j = i++
+    j.hashCode()
+    return i
+}