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

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

@@ -0,0 +1,12 @@
+class MyClass
+
+// Correct at compile time but wrong at run-time
+operator fun MyClass?.inc(): MyClass? { return null }
+
+public fun box() : MyClass? {
+    var i : MyClass? 
+    i = MyClass()
+    var j = i++
+    j.hashCode()
+    return i
+}