i965/fs: Fix off-by-one in saturate propagation.

i965/fs: Fix off-by-one in saturate propagation.

ip needs to be initialized to start_ip - 1, since the first thing in the
main loop is ip++. Otherwise we would incorrectly propagate the saturate
from the mov to the mad:

   mad     a, b, c, d
   mov.sat x, a
   add     y, z, a

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>