nir/algebraic: Simplify logic to detect sign of an integer

nir/algebraic: Simplify logic to detect sign of an integer

This occurs in a handful of places in the soft-fp64 code, and that is
the primary reason for the change.

v2: Fix a typo in a comment.  Noticed by Matt.  Copy the correct fp64
shader-db results to the commit message.  I realized that I used
accidentally used the results from the next commit.

Results on the 308 shaders extracted from the fp64 portion of the OpenGL
CTS:

Tiger Lake and Ice Lake had similar results. (Tiger Lake shown)
total instructions in shared programs: 906235 -> 906149 (<.01%)
instructions in affected programs: 353966 -> 353880 (-0.02%)
helped: 31
HURT: 2
helped stats (abs) min: 1 max: 8 x̄: 3.03 x̃: 3
helped stats (rel) min: 0.01% max: 1.59% x̄: 0.10% x̃: 0.04%
HURT stats (abs)   min: 3 max: 5 x̄: 4.00 x̃: 4
HURT stats (rel)   min: 0.02% max: 0.02% x̄: 0.02% x̃: 0.02%
95% mean confidence interval for instructions value: -3.51 -1.70
95% mean confidence interval for instructions %-change: -0.19% <.01%
Inconclusive result (%-change mean confidence interval includes 0).

total cycles in shared programs: 7076552 -> 7076173 (<.01%)
cycles in affected programs: 2878361 -> 2877982 (-0.01%)
helped: 37
HURT: 2
helped stats (abs) min: 2 max: 48 x̄: 10.81 x̃: 6
helped stats (rel) min: <.01% max: 2.17% x̄: 0.47% x̃: 0.01%
HURT stats (abs)   min: 1 max: 20 x̄: 10.50 x̃: 10
HURT stats (rel)   min: <.01% max: 0.01% x̄: <.01% x̃: <.01%
95% mean confidence interval for cycles value: -13.96 -5.48
95% mean confidence interval for cycles %-change: -0.72% -0.16%
Cycles are helped.

total fills in shared programs: 2064 -> 2065 (0.05%)
fills in affected programs: 45 -> 46 (2.22%)
helped: 0
HURT: 1

Regular shader-db results:

All Gen7+ platforms had similar results. (Tiger Lake shown)
total instructions in shared programs: 17611530 -> 17611506 (<.01%)
instructions in affected programs: 5934 -> 5910 (-0.40%)
helped: 10
HURT: 0
helped stats (abs) min: 1 max: 5 x̄: 2.40 x̃: 2
helped stats (rel) min: 0.14% max: 1.24% x̄: 0.47% x̃: 0.34%
95% mean confidence interval for instructions value: -3.53 -1.27
95% mean confidence interval for instructions %-change: -0.78% -0.17%
Instructions are helped.

total cycles in shared programs: 338419178 -> 338419218 (<.01%)
cycles in affected programs: 19244 -> 19284 (0.21%)
helped: 4
HURT: 2
helped stats (abs) min: 2 max: 4 x̄: 3.00 x̃: 3
helped stats (rel) min: 0.05% max: 0.11% x̄: 0.08% x̃: 0.08%
HURT stats (abs)   min: 26 max: 26 x̄: 26.00 x̃: 26
HURT stats (rel)   min: 1.20% max: 1.20% x̄: 1.20% x̃: 1.20%
95% mean confidence interval for cycles value: -9.08 22.41
95% mean confidence interval for cycles %-change: -0.35% 1.04%
Inconclusive result (value mean confidence interval includes 0).

No changes on any earlier Intel platform.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4142>