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/postfixNullableIncrement.kt

@@ -0,0 +1,10 @@
+fun Int?.inc(): Int? = this
+
+fun init(): Int? { return 10 }
+
+public fun box() : String {
+    var i : Int? = init()
+    val j = i++
+
+    return if (j == 10 && 10 == i) "OK" else "fail i = $i j = $j"
+}