Postfix expression code generation fixed as per KT-7561.

A set of tests for KT-7561 on codegen side, including exotic inc() functions().
 #KT-7561 Fixed.

compiler/testData/codegen/box/increment/postfixNotnullClassIncrement.kt

@@ -0,0 +1,12 @@
+class MyClass
+
+// In principle it is not correct, MyClass? is not a subtype of MyClass
+fun MyClass.inc(): MyClass? = null
+
+public fun box() : String {
+    var i : MyClass? 
+    i = MyClass()
+    val j = i++
+
+    return if (j is MyClass && null == i) "OK" else "fail i = $i j = $j"
+}