soft-fp64/fsat: Micro-optimize x < 0 test

soft-fp64/fsat: Micro-optimize x < 0 test

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: 841647 -> 841590 (<.01%)
instructions in affected programs: 122014 -> 121957 (-0.05%)
helped: 7
HURT: 0
helped stats (abs) min: 3 max: 12 x̄: 8.14 x̃: 9
helped stats (rel) min: 0.04% max: 0.07% x̄: 0.05% x̃: 0.04%
95% mean confidence interval for instructions value: -11.23 -5.06
95% mean confidence interval for instructions %-change: -0.06% -0.03%
Instructions are helped.

total cycles in shared programs: 6926904 -> 6926828 (<.01%)
cycles in affected programs: 1038645 -> 1038569 (<.01%)
helped: 7
HURT: 0
helped stats (abs) min: 4 max: 16 x̄: 10.86 x̃: 12
helped stats (rel) min: <.01% max: 0.01% x̄: <.01% x̃: <.01%
95% mean confidence interval for cycles value: -14.97 -6.74
95% mean confidence interval for cycles %-change: -0.01% <.01%
Cycles are helped.

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>