As Samuel Irving “S.I.” Newhouse Jr. assembled one of the great private art collections of the modern era, he relied on the same instincts that helped build his hugely successful publishing empire — a demand for quality and a willingness to pay whatever it took for the very best. Now, nearly a decade after his death, a significant portion of that collection is heading to auction. Christie’s will sell 16 works from the private collection of S.I. Newhouse on May 18 in what is expected to become