UBS' top equity strategist dialed back his view on U.S. stocks, citing mounting risks from a weakening dollar, stretched valuations and policy turbulence in Washington. Andrew Garthwaite, head of global equity strategy at the investment bank, downgraded American equities to "benchmark" in a fully invested global equity portfolio, arguing that the factors that powered years of outperformance are starting to fade. The dollar risk is a central concern, Garthwaite wrote. UBS forecasts the euro