A rival is on the fiscal ropes, and could be partially (or even wholly) going away in the near future. Shares of Frontier Airlines parent Frontier Group Holdings (ULCC 18.34%) are up 18% as of midday Friday, followed closely by JetBlue Airways' (JBLU 14.08%) 16% surge. Both rallies are in response to reports that rival carrier Spirit Airlines (SAVE -26.56%) is considering filing for bankruptcy. If true, it could mean the sale of part or all of the struggling airline. Bankruptcy imminent? Take the report wit ...