Compiler update

The __builtin_rvtt_sfpswap builtin has been corrected. It now returns a longer vector type that may be inspected with a new __builtin_rvtt_sfpselect2 builtin. For instance:

  auto r = __builtin_rvtt_sfpswap (a, b, mod);
  auto ra = __builtin_rvtt_select2 (r, 0);
  auto rb = __builtin_rvtt_select2 (r, 1);

This change is necessary to properly represent the data flow within the compiler.

The sfpi library's vec_swap and vec_min_max functions have been adjusted.