Dexcom Inc DXCM.O jumped after the glucose monitoring systems maker denied a report on merger talks with insulin pump maker Insulet Corp PODD.O. By Sinéad Carew and Anisha Sircar May 31 (Reuters) - The S&P 500 fell slightly on Tuesday after a three-session rally as volatile trading in oil prices kept soaring inflation in focus and investors reacted to hawkish comments from a Federal Reserve official. Fed Governor Christopher Waller said on Monday the U.S. central bank should be prepared to raise rates by a half percentage point at every meeting from now on until inflation is decisively curbed.