U.S. sanctions have cut off Iranian oil shipments to Syria, taking an unprecedented toll on a flow of crude that had persisted in the face of long-term international restrictions and helped sustain the Assad regime through years of civil war. Iran has been unable to deliver oil to Syria since Jan. 2., according to maritime-data provider TankerTrackers.com. That compares with an average of 66,000 barrels a day in the three months to the New Year. Iran supplies have “fallen off the cliff” and storage tanks are virtually empty…