soft-fp64: Relax the way NaN is propagated

soft-fp64: Relax the way NaN is propagated

Also reassociate a couple expressions to encourage some CSE.

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: 813599 -> 797951 (-1.92%)
instructions in affected programs: 796110 -> 780462 (-1.97%)
helped: 92
HURT: 0
helped stats (abs) min: 3 max: 5198 x̄: 170.09 x̃: 83
helped stats (rel) min: 0.36% max: 5.50% x̄: 1.57% x̃: 1.40%
95% mean confidence interval for instructions value: -282.42 -57.75
95% mean confidence interval for instructions %-change: -1.71% -1.42%
Instructions are helped.

total cycles in shared programs: 6687128 -> 6601437 (-1.28%)
cycles in affected programs: 6582246 -> 6496555 (-1.30%)
helped: 92
HURT: 0
helped stats (abs) min: 36 max: 14442 x̄: 931.42 x̃: 592
helped stats (rel) min: 0.45% max: 3.16% x̄: 1.44% x̃: 1.23%
95% mean confidence interval for cycles value: -1257.58 -605.27
95% mean confidence interval for cycles %-change: -1.58% -1.30%
Cycles are helped.

total spills in shared programs: 759 -> 702 (-7.51%)
spills in affected programs: 759 -> 702 (-7.51%)
helped: 3
HURT: 0

total fills in shared programs: 2412 -> 1442 (-40.22%)
fills in affected programs: 2412 -> 1442 (-40.22%)
helped: 3
HURT: 0

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