nir: add option to lower slt/sge/seq/sne

nir: add option to lower slt/sge/seq/sne

In freedreno these get implemented as the matching f* instruction plus a
u2f to convert the result to float 1.0/0.0.  But less lines of code to
just let nir_opt_algebraic handle this for us, plus opens up some small
window for other opt passes to improve (ie. if some shader ended up with
both a flt and slt with same src args, for example).

v2: use b2f rather than u2f

Signed-off-by: Rob Clark <robclark@freedesktop.org>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>