nir/algebraic: Simplify some Inf and NaN avoidance code

nir/algebraic: Simplify some Inf and NaN avoidance code

Since a is non-negative, neither fsqrt nor frsq should return NaN.  frsq
should only return Inf when fsqrt returns 0.

The changes are pretty small, but this turns a few hundred hurt shaders
in the next patch into helped shaders.

An alternative to the intBitsToFloat is to import numpy and do
np.finfo(np.float32).max.  That's more explicit, but we may also want to
have specific bit encodings of float values later.  I could be convinced
either way, but intBitsToFloat(0x7f7fffff) was what I implemented first.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
All Gen7+ platforms had similar results. (Ice Lake shown)
total instructions in shared programs: 14661140 -> 14661104 (<.01%)
instructions in affected programs: 7520 -> 7484 (-0.48%)
helped: 36
HURT: 0
helped stats (abs) min: 1 max: 1 x̄: 1.00 x̃: 1
helped stats (rel) min: 0.32% max: 0.61% x̄: 0.49% x̃: 0.52%
95% mean confidence interval for instructions value: -1.00 -1.00
95% mean confidence interval for instructions %-change: -0.52% -0.47%
Instructions are helped.

total cycles in shared programs: 228585416 -> 228584806 (<.01%)
cycles in affected programs: 56321 -> 55711 (-1.08%)
helped: 32
HURT: 0
helped stats (abs) min: 2 max: 98 x̄: 19.06 x̃: 10
helped stats (rel) min: 0.08% max: 6.41% x̄: 1.09% x̃: 0.65%
95% mean confidence interval for cycles value: -28.32 -9.80
95% mean confidence interval for cycles %-change: -1.63% -0.54%
Cycles are helped.

Sandy Bridge
total cycles in shared programs: 152991077 -> 152991075 (<.01%)
cycles in affected programs: 11525 -> 11523 (-0.02%)
helped: 2
HURT: 2
helped stats (abs) min: 2 max: 4 x̄: 3.00 x̃: 3
helped stats (rel) min: 0.07% max: 0.11% x̄: 0.09% x̃: 0.09%
HURT stats (abs)   min: 2 max: 2 x̄: 2.00 x̃: 2
HURT stats (rel)   min: 0.08% max: 0.08% x̄: 0.08% x̃: 0.08%
95% mean confidence interval for cycles value: -5.27 4.27
95% mean confidence interval for cycles %-change: -0.16% 0.15%
Inconclusive result (value mean confidence interval includes 0).

No changes on Iron Lake or GM45.