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/postfixIncrementOnClass.kt

@@ -0,0 +1,12 @@
+trait Base
+class Derived: Base
+class Another: Base
+fun Base.inc(): Derived { return Derived() }
+
+public fun box() : String {
+    var i : Base
+    i = Another()
+    val j = i++
+
+    return if (j is Another && i is Derived) "OK" else "fail j = $j i = $i"
+}