Stocks End Up; Merck Helps Dow; Dollar Up  NEW YORK (Reuters) - U.S. stocks closed higher on  Wednesday, as the dollar strengthened and shares of Merck   Co.  &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=MRK.N target=/stocks/quickinfo/fullquote"&gt;MRK.N&lt;/A&gt; helped lift the Dow after the drugmaker gave a forecast  that was not as weak as some on Wall Street had expected.