Simpson Thacher & Bartlett and Cleary Gottlieb Steen & Hamilton were outside counsel on the biggest US IPO of the year so far. Credit card operator Mastercard raised $2.4 billion when it listed on the NYSE. Vincent Pagano and Joshua Ford Bonnie led for Simpson Thacher, which represented Mastercard. Cleary Gottlieb's David Lopez was adviser to the underwriters Goldman Sachs....