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/prefixNullableClassIncrement.kt

@@ -0,0 +1,13 @@
+class MyClass
+
+// Correct at compile time but wrong at run-time
+fun MyClass?.inc(): MyClass? { return null }
+
+public fun box() {
+    var i : MyClass? 
+    i = MyClass()
+    // type of j should be MyClass?
+    var j = ++i
+    // j is null so call should be unsafe
+    j<!UNSAFE_CALL!>.<!>hashCode()
+}