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/OperationWhen.kt

@@ -0,0 +1,20 @@
+sealed class Operation(val left: Int, val right: Int) {
+    abstract fun exec(): Int
+    class Plus(left: Int, right: Int): Operation(left, right) {
+        override fun exec(): Int = left + right
+    }
+    class Minus(left: Int, right: Int): Operation(left, right) {
+        override fun exec(): Int = left - right
+    }
+    class Times(left: Int, right: Int): Operation(left, right) {
+        override fun exec(): Int = left * right
+    }
+    class Slash(left: Int, right: Int): Operation(left, right) {
+        override fun exec(): Int = left / right
+    }
+}
+
+fun priority(op: Operation) = when(op) {
+    is Operation.Plus, is Operation.Minus  -> 1
+    is Operation.Times, is Operation.Slash -> 2
+}