soft-fp64/fsat: Correctly handle NaN

soft-fp64/fsat: Correctly handle NaN

fsat is defined as min(max(a, 0.0), 1.0), and IEEE defines both min and
max to return the non-NaN value when one value is NaN.  Based on this,
fsat should definitely return 0.0 for NaN.

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: 841666 -> 841647 (<.01%)
instructions in affected programs: 122033 -> 122014 (-0.02%)
helped: 7
HURT: 0
helped stats (abs) min: 1 max: 4 x̄: 2.71 x̃: 3
helped stats (rel) min: 0.01% max: 0.02% x̄: 0.02% x̃: 0.01%
95% mean confidence interval for instructions value: -3.74 -1.69
95% mean confidence interval for instructions %-change: -0.02% -0.01%
Instructions are helped.

total cycles in shared programs: 6927246 -> 6926904 (<.01%)
cycles in affected programs: 1038987 -> 1038645 (-0.03%)
helped: 7
HURT: 0
helped stats (abs) min: 18 max: 72 x̄: 48.86 x̃: 54
helped stats (rel) min: 0.03% max: 0.05% x̄: 0.03% x̃: 0.03%
95% mean confidence interval for cycles value: -67.38 -30.33
95% mean confidence interval for cycles %-change: -0.05% -0.02%
Cycles are helped.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Fixes: a42163cbbc1 ("compiler: Add lowering support for 64-bit saturate operations to software")
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2585
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4142>