Put not-null original type as last possible smart cast target

This fixes muted tests testSmartCast (IDE highlighting) and
testNestedSealed (Diagnostics).
Together with commit 555b3f12 this makes #KT-15901 Fixed

idea/testData/checker/infos/SmartCastTarget.kt

@@ -0,0 +1,14 @@
+// See KT-15901
+
+<info>enum</info> class En { A }
+
+fun foo(): Any {
+    val en2: Any? = En.A
+    if (en2 is En) {
+        // Here we had smart casts to En / Any
+        // should be always En
+        val a: Any = <info descr="Smart cast to En">en2</info>
+        return a
+    }
+    return ""
+}