KT-22274 report warning on labels that can't be referenced

Labels are meaningful only if they can be referenced by 'break',
'continue', or 'return' expressions.

compiler/testData/diagnostics/tests/Casts.kt

@@ -17,7 +17,7 @@ fun test() : Unit {
 
   val <!UNUSED_VARIABLE!>s<!> = "" as Any
   ("" as String?)?.length
-  (data@("" as String?))?.length
+  (<!REDUNDANT_LABEL_WARNING!>data@<!>("" as String?))?.length
   (<!WRONG_ANNOTATION_TARGET!>@MustBeDocumented()<!>( "" as String?))?.length
   Unit
 }