SUSAN MATHEW FRAYED nerves ahead of U.S. inflation data later this week cast a shadow over emerging markets on Tuesday, with South Africa's rand leading losses ahead of domestic economic growth data. Against a stronger dollar, the rand fell 0.6%, pulling further away from 28-month highs. GDP numbers later in the morning are expected to show the South African economy slowed in the first quarter. But the rand is still by far the best performing EM currency this year, up more than 8%, helped in part by rising commodity prices. Turkey's lira – the year's worst performing EM currency so…