METALS-Shanghai base metals mixed ahead of U.S.-China trade talks

(Adds analyst comment, updates prices) BEIJING, May 3 (Reuters) - Shanghai base metals were mixed on Thursday as the market fretted about the outcome of Sino-U.S. trade talks that would start later in Beijing. The world's two biggest economies have imposed import tariffs on each other's goods, including Chinese aluminium and U.S. aluminium scrap, and threatened more action in a trade dispute that has roiled metals markets. Shanghai aluminium, nickel and lead crept higher, but copper