There should be an option to undo your comment upvote if a comment gets edited. It is likely that you upvote something else which gets useless or provides incorrect information after the edit.
I haven't had very many cases where I would have wanted this.
Either it doesn't matter too much, just like an incorrect upvote or downvote on a post, or there is another way out (flagging if the edit is really terrible). And comments are second class citizens; they don't have any influence on reputation and they can be removed at will of the moderator, so no need to worry too much about them.
All these points combined shows that there is no need for this feature.
While it may not be the most critical feature for Stack Exchange, I agree it would be nice to have.
In addition to giving the option to undo an upvote on edited comments, I'm a fan of this idea on a similar post. You could let the user know a comment they have voted on has been edited by sending a notification.