glsl: Implement usubBorrow() built-in for ARB_gpu_shader5.

glsl: Implement usubBorrow() built-in for ARB_gpu_shader5.

i965 implements this with a single (multiple destination) instruction,
SUBB. Emitting SUBB directly from usubBorrow() would be ideal, but our
optimization passes don't know how to copy with expressions with
side-effects.

Radeon has an SUBB_UINT instruction that only generates the borrow
bit. I've chosen to go this route and implement usubBorrow() by doing the
subtraction and the borrow operations separately.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>