Front-end fix: result type corrected for increment / decrement expressions. Smart cast allowed on a postfix increment result. See KT-7561.

Now result type is receiver type for postfix increment, or increment result type for prefix increment.
A set of relevant tests.

compiler/testData/diagnostics/tests/smartCasts/varnotnull/postfixNullableIncrement.kt

@@ -0,0 +1,8 @@
+fun Int?.inc(): Int? { return this }
+
+public fun box(arg: Int?) : Int? {
+    var i : Int? = arg
+    var j = i++
+    j<!UNSAFE_CALL!>.<!>toInt()
+    return i
+}