Being asked to review a paper in conference one has submitted to

No, there is no conflict. Your advice will be backed up or not by other reviewers. Likely the conference committee already knows that you are also a submitter.

But if you want double assurance of this, just send a note to the program chair that you have also submitted. If they see any issue, they will deal with it.

Just give an honest review as you would in any case. And trust that your paper will be accepted or not on its merits. Of course, there is quite a lot of competition, but that is always true.

I see no reason why it would be a conflict of interest. An analogous question would be if there is a conflict of interest in you reviewing papers in a journal you might publish in. Given that other conference attendees are in the same/similar field as you, and are interested in a good conference, those attendees would seem to be the ideal group to pull reviewers from.

In conferences where I've seen the sausage being made, accepting to review gains no advantage. Refusing any and all review requests will ultimately be viewed as rude, but will not disadvantage you in having your paper accepted. (Invitations for invited talks and membership on the conference committee may well be hindered, however.)