[IR] Extend test coverage for smart cast handling.

compiler/testData/ir/irText/expressions/whenSmartCastToEnum.kt

@@ -0,0 +1,23 @@
+enum class En { A, B, C }
+
+fun test() {
+    var r = ""
+
+    val x: Any? = En.A
+    if (x is En) {
+        when (x) {
+            En.A -> { r = "when1" }
+            En.B -> {}
+            En.C -> {}
+        }
+    }
+
+    val y: Any = En.A
+    if (y is En) {
+        when (y) {
+            En.A -> { r = "when2" }
+            En.B -> {}
+            En.C -> {}
+        }
+    }
+}