soft-fp64/fneg: Don't treat NaN specially

__fabs64 doesn't do anything special, and the value is still NaN
regardless of the value of the MSB.  In a strict sense, it's possible
that both functions should set the "signal" bit.

lts 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: 844558 -> 843005 (-0.18%)
instructions in affected programs: 725975 -> 724422 (-0.21%)
helped: 53
HURT: 4
helped stats (abs) min: 1 max: 313 x̄: 29.87 x̃: 21
helped stats (rel) min: 0.01% max: 0.94% x̄: 0.30% x̃: 0.22%
HURT stats (abs)   min: 4 max: 11 x̄: 7.50 x̃: 7
HURT stats (rel)   min: 0.03% max: 0.09% x̄: 0.05% x̃: 0.04%
95% mean confidence interval for instructions value: -39.02 -15.47
95% mean confidence interval for instructions %-change: -0.34% -0.21%
Instructions are helped.

total cycles in shared programs: 6962024 -> 6944998 (-0.24%)
cycles in affected programs: 6185470 -> 6168444 (-0.28%)
helped: 59
HURT: 0
helped stats (abs) min: 64 max: 2863 x̄: 288.58 x̃: 208
helped stats (rel) min: 0.11% max: 0.87% x̄: 0.33% x̃: 0.27%
95% mean confidence interval for cycles value: -387.15 -190.00
95% mean confidence interval for cycles %-change: -0.38% -0.28%
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>

src/compiler/glsl/float64.glsl

@@ -88,10 +88,7 @@ uint64_t
 __fneg64(uint64_t __a)
 {
    uvec2 a = unpackUint2x32(__a);
-   uint t = a.y;
-
-   t ^= (1u << 31);
-   a.y = mix(t, a.y, __is_nan(__a));
+   a.y ^= (1u << 31);
    return packUint2x32(a);
 }