Prevent expression selector from selecting just a label

Previously, extract variable could act on e.g. 'break@outerLoop' and
extract the label, leading into nonsensical 'val x = @outerLoop'.
Prevent JetRefactoringUtil.selectExpression from returning label
expressions to fix this.

 #KT-4515 Fixed

idea/testData/smartSelection/labelledStatement.kt

@@ -0,0 +1,7 @@
+fun f() {
+    @loop while (true) {
+        break@lo<caret>op
+    }
+}
+// KT-4515: Extract variable can attempt to extract a label from a labelled statement.
+/* */