NEW YORK, April 15 : The U.S. dollar has given back most of the gains sparked by the Iran war, as a tentative ceasefire revived appetite for riskier currencies, but investors said robust demand for U.S. assets and waning prospects for U.S. interest-rate cuts should buttress it against sharper declines. The dollar index, which measures the currency's strength against six major peers, rose more than 3 per cent to a 10-month high of 100.64 after the U.S.-Iran conflict ignited a rush for safe