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/postfixIncrementOnClassSmartCast.kt

@@ -0,0 +1,11 @@
+open class Base
+class Derived: Base()
+fun Derived.inc(): Derived { return Derived() }
+
+public fun box() : String {
+    var i : Base
+    i = Derived()
+    val j = i++
+
+    return if (j is Derived && i is Derived) "OK" else "fail j = $j i = $i"
+}