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/prefixNotnullClassIncrement.kt

@@ -0,0 +1,13 @@
+class MyClass
+
+// In principle it is not correct, MyClass? is not a subtype of MyClass
+fun MyClass.inc(): MyClass? { return null }
+
+public fun box() {
+    var i : MyClass? 
+    i = MyClass()
+    // Type of j should be inferred as MyClass?
+    var j = ++<!DEBUG_INFO_SMARTCAST!>i<!>
+    // j is null so call is unsafe
+    j<!UNSAFE_CALL!>.<!>hashCode()
+}