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

compiler/frontend/src/org/jetbrains/jet/lang/psi/JetPsiUtil.java

@@ -1042,4 +1042,9 @@ public class JetPsiUtil {
                 expression;
         return (JetToken) elementType;
     }
+
+    public static boolean isLabelIdentifierExpression(PsiElement element) {
+        return element instanceof JetSimpleNameExpression &&
+               ((JetSimpleNameExpression) element).getReferencedNameElementType() == JetTokens.LABEL_IDENTIFIER;
+    }
 }