FIR checker: warn useless elvis

compiler/testData/diagnostics/tests/smartCasts/elvisExprNotNull.fir.kt

@@ -31,7 +31,7 @@ fun baz(s: String?, r: String?): String {
 }
 
 fun withNull(s: String?): String {
-    val t = s ?: null
+    val t = s <!USELESS_ELVIS_RIGHT_IS_NULL!>?: null<!>
     // Error: nullable
     return <!RETURN_TYPE_MISMATCH!>t<!>
-}
+}