swr: [rasterizer jitter] fix llvm >= 5.0 build break

swr: [rasterizer jitter] fix llvm >= 5.0 build break

Function::getArgumentList() doesn't exist anymore, switch to using
arg_begin() (existed back to at least llvm-3.6.0).

Reviewed-by: Vedran Miletić <vedran@miletic.net>
CC: <mesa-stable@lists.freedesktop.org>