Krispy Kreme Stock Tumbles Amid Probe  NEW YORK (Reuters) - U.S. securities regulators have  ratcheted up their probe of Krispy Kreme Doughnuts Inc.  &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=KKD.N target=/stocks/quickinfo/fullquote"&gt;KKD.N&lt;/A&gt;, looking at its franchise repurchases, the company said  on Friday, triggering a slide of nearly 12 percent in its stock  before the bell.