Albemarle Corp ALB.N and Livent Corp LTHM.N fell 4.5% and 6.6%, respectively, after Goldman Sachs downgraded both lithium producers to "sell" from "neutral". By Shreyashi Sanyal Dec 15 (Reuters) - U.S. stock index futures were subdued on Wednesday as the latest reading on inflation solidified bets that the Federal Reserve might announce a speedier wind down of its pandemic-era monetary stimulus later in the day. Data on Tuesday showed producer prices increased more than expected in the 12 months through November, clocking its largest gain since 2010 and further adding pressure on the Fed.