nir/algebraic: Optimize some cases of (sXX(a, b) != 0.0)

nir/algebraic: Optimize some cases of (sXX(a, b) != 0.0)

I noticed the SGE case while looking at the output of
shaders/closed/steam/trine-2/fp-3.shader_test on i915g.  These are
especially bad on i915 that needs two instructions to implement SNE.

An alternative would be to duplicate the sne(sXX(a, b), 0.0) rules in an
algebraic pass that occurs after bool_to_float.  Doing the work earlier
seems preferable.

i915
total instructions in shared programs: 788274 -> 788223 (<.01%)
instructions in affected programs: 666 -> 615 (-7.66%)
helped: 5
HURT: 0
helped stats (abs) min: 9 max: 12 x̄: 10.20 x̃: 9
helped stats (rel) min: 5.00% max: 11.11% x̄: 8.12% x̃: 8.16%
95% mean confidence interval for instructions value: -12.24 -8.16
95% mean confidence interval for instructions %-change: -10.81% -5.43%
Instructions are helped.

LOST:   0
GAINED: 2

The two gained shaders are assembly fragment programs in Euro Truck
Simulator 2.

Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15210>