nir/algebraic: Convert some f2u to f2i

nir/algebraic: Convert some f2u to f2i

Section 5.4.1 (Conversion and Scalar Constructors) of the GLSL 4.60 spec
says:

     It is undefined to convert a negative floating-point value to an
     uint.

Assuming that (uint)some_float behaves like (uint)(int)some_float allows
some optimizations in the i965 backend to proceed.

This basically undoes the small amount of damage done by
"intel/compiler: Avoid propagating inequality cmods if types are
different".

v2: Replicate part of the commit message as a comment in the code.
Suggested by Jason.

shader-db results compairing *before* "intel/compiler: Avoid propagating
inequality cmods if types are different" and after this commit:

Skylake
total cycles in shared programs: 383007996 -> 383007896 (<.01%)
cycles in affected programs: 85208 -> 85108 (-0.12%)
helped: 13
HURT: 8
helped stats (abs) min: 2 max: 26 x̄: 10.77 x̃: 6
helped stats (rel) min: 0.09% max: 0.65% x̄: 0.28% x̃: 0.14%
HURT stats (abs)   min: 2 max: 12 x̄: 5.00 x̃: 3
HURT stats (rel)   min: 0.04% max: 0.32% x̄: 0.12% x̃: 0.07%
95% mean confidence interval for cycles value: -9.31 -0.21
95% mean confidence interval for cycles %-change: -0.24% <.01%
Cycles are helped.

Broadwell
total cycles in shared programs: 415251194 -> 415251370 (<.01%)
cycles in affected programs: 83750 -> 83926 (0.21%)
helped: 7
HURT: 13
helped stats (abs) min: 10 max: 12 x̄: 11.43 x̃: 12
helped stats (rel) min: 0.30% max: 0.30% x̄: 0.30% x̃: 0.30%
HURT stats (abs)   min: 2 max: 36 x̄: 19.69 x̃: 22
HURT stats (rel)   min: 0.05% max: 0.89% x̄: 0.44% x̃: 0.47%
95% mean confidence interval for cycles value: 0.76 16.84
95% mean confidence interval for cycles %-change: <.01% 0.37%
Inconclusive result (%-change mean confidence interval includes 0).

Haswell
total instructions in shared programs: 13823885 -> 13823886 (<.01%)
instructions in affected programs: 2249 -> 2250 (0.04%)
helped: 0
HURT: 1

total cycles in shared programs: 390094243 -> 390094001 (<.01%)
cycles in affected programs: 85640 -> 85398 (-0.28%)
helped: 15
HURT: 6
helped stats (abs) min: 4 max: 26 x̄: 18.53 x̃: 18
helped stats (rel) min: 0.09% max: 0.66% x̄: 0.47% x̃: 0.42%
HURT stats (abs)   min: 2 max: 14 x̄: 6.00 x̃: 2
HURT stats (rel)   min: 0.04% max: 0.37% x̄: 0.15% x̃: 0.04%
95% mean confidence interval for cycles value: -17.36 -5.69
95% mean confidence interval for cycles %-change: -0.44% -0.14%
Cycles are helped.

Ivy Bridge
total cycles in shared programs: 180986448 -> 180986552 (<.01%)
cycles in affected programs: 34835 -> 34939 (0.30%)
helped: 0
HURT: 10
HURT stats (abs)   min: 2 max: 18 x̄: 10.40 x̃: 10
HURT stats (rel)   min: 0.06% max: 0.36% x̄: 0.28% x̃: 0.30%
95% mean confidence interval for cycles value: 4.67 16.13
95% mean confidence interval for cycles %-change: 0.20% 0.35%
Cycles are HURT.

Sandy Bridge
total cycles in shared programs: 154603969 -> 154603970 (<.01%)
cycles in affected programs: 171514 -> 171515 (<.01%)
helped: 25
HURT: 14
helped stats (abs) min: 1 max: 4 x̄: 1.80 x̃: 1
helped stats (rel) min: 0.02% max: 0.10% x̄: 0.04% x̃: 0.04%
HURT stats (abs)   min: 1 max: 8 x̄: 3.29 x̃: 3
HURT stats (rel)   min: 0.03% max: 0.28% x̄: 0.10% x̃: 0.11%
95% mean confidence interval for cycles value: -0.91 0.96
95% mean confidence interval for cycles %-change: -0.02% 0.04%
Inconclusive result (value mean confidence interval includes 0).

No changes on Iron Lake or GM45.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>