Exhaustive when support for sealed classes #KT-7606 Fixed

When on sealed can use is (both for derived classes and objects) or just comparison (only for derived objects).
A pack of tests provided.

compiler/testData/diagnostics/tests/sealed/TreeWhen.kt

@@ -0,0 +1,13 @@
+sealed class Tree {
+    object Empty: Tree()
+    class Leaf(val x: Int): Tree()
+    class Node(val left: Tree, val right: Tree): Tree()
+
+    fun max(): Int {
+        when(this) {
+            is Empty -> return -1
+            is Leaf -> return <!DEBUG_INFO_SMARTCAST!>this<!>.x
+            is Node -> return <!DEBUG_INFO_SMARTCAST!>this<!>.left.max()
+        }
+    }
+}