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/postfixNullableClassIncrement.kt

@@ -0,0 +1,12 @@
+class MyClass
+
+// Correct at compile time but wrong at run-time
+fun MyClass?.inc(): MyClass? { return null }
+
+public fun box() : MyClass? {
+    var i : MyClass? 
+    i = MyClass()
+    var j = i++
+    <!DEBUG_INFO_SMARTCAST!>j<!>.hashCode()
+    return i
+}