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/postfixIncrementOnSmartCast.kt

@@ -0,0 +1,9 @@
+public fun box() : String {
+    var i : Int?
+    i = 10
+    // Postfix increment on a smart cast should work
+    // Specific: i.inc() type is Int but i and j types are both Int?
+    val j = i++
+
+    return if (j == 10 && 11 == i) "OK" else "fail j = $j i = $i"
+}