soft-fp64/flt: Perform checks in a different order

soft-fp64/flt: Perform checks in a different order

The change to nir_opt_algebraic cleans up a pattern that was never
produced before the rest of this commit was added.

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: 843005 -> 841666 (-0.16%)
instructions in affected programs: 460655 -> 459316 (-0.29%)
helped: 64
HURT: 17
helped stats (abs) min: 1 max: 72 x̄: 21.72 x̃: 20
helped stats (rel) min: 0.01% max: 28.07% x̄: 12.67% x̃: 16.07%
HURT stats (abs)   min: 1 max: 7 x̄: 3.00 x̃: 2
HURT stats (rel)   min: 0.01% max: 0.04% x̄: 0.02% x̃: 0.02%
95% mean confidence interval for instructions value: -20.87 -12.19
95% mean confidence interval for instructions %-change: -12.35% -7.66%
Instructions are helped.

total cycles in shared programs: 6944998 -> 6927246 (-0.26%)
cycles in affected programs: 3891872 -> 3874120 (-0.46%)
helped: 71
HURT: 10
helped stats (abs) min: 2 max: 772 x̄: 254.21 x̃: 156
helped stats (rel) min: <.01% max: 66.44% x̄: 21.72% x̃: 18.40%
HURT stats (abs)   min: 18 max: 69 x̄: 29.70 x̃: 20
HURT stats (rel)   min: 0.02% max: 0.04% x̄: 0.03% x̃: 0.03%
95% mean confidence interval for cycles value: -270.82 -167.50
95% mean confidence interval for cycles %-change: -24.41% -13.65%
Cycles are helped.

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