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/assignPlusOnSmartCast.kt

@@ -0,0 +1,8 @@
+public fun box() : String {
+    var i : Int?
+    i = 10
+    // assignPlus on a smart cast should work
+    i += 1
+
+    return if (11 == i) "OK" else "fail i = $i"
+}