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/postfixIncrementOnShortSmartCast.kt

@@ -0,0 +1,8 @@
+public fun box() : String {
+    var i : Short?
+    i = 10
+    // Postfix increment on a smart casted short should work
+    val j = i++
+
+    return if (j!!.toInt() == 10 && i!!.toInt() == 11) "OK" else "fail j = $j i = $i"
+}