nir: Don't compare i2f or u2i with zero

nir: Don't compare i2f or u2i with zero

Broadwell and Skylake had similar results. (Skylake shown)
total instructions in shared programs: 14277620 -> 14277230 (<.01%)
instructions in affected programs: 36905 -> 36515 (-1.06%)
helped: 101
HURT: 6
helped stats (abs) min: 1 max: 6 x̄: 4.46 x̃: 6
helped stats (rel) min: 0.32% max: 7.69% x̄: 1.80% x̃: 1.51%
HURT stats (abs)   min: 1 max: 28 x̄: 10.00 x̃: 1
HURT stats (rel)   min: 0.33% max: 1.74% x̄: 0.68% x̃: 0.47%
95% mean confidence interval for instructions value: -4.59 -2.70
95% mean confidence interval for instructions %-change: -1.90% -1.41%
Instructions are helped.

total cycles in shared programs: 532580716 -> 532577947 (<.01%)
cycles in affected programs: 940575 -> 937806 (-0.29%)
helped: 92
HURT: 12
helped stats (abs) min: 2 max: 158 x̄: 51.04 x̃: 62
helped stats (rel) min: 0.24% max: 3.99% x̄: 2.14% x̃: 2.41%
HURT stats (abs)   min: 10 max: 1112 x̄: 160.58 x̃: 63
HURT stats (rel)   min: 0.06% max: 21.90% x̄: 4.22% x̃: 0.20%
95% mean confidence interval for cycles value: -50.66 -2.59
95% mean confidence interval for cycles %-change: -2.09% -0.73%
Cycles are helped.

total spills in shared programs: 8116 -> 8124 (0.10%)
spills in affected programs: 200 -> 208 (4.00%)
helped: 0
HURT: 2

total fills in shared programs: 11086 -> 11094 (0.07%)
fills in affected programs: 436 -> 444 (1.83%)
helped: 0
HURT: 2

Ivy Bridge and Haswell had similar results. (Haswell shown)
total instructions in shared programs: 12979054 -> 12978067 (<.01%)
instructions in affected programs: 33633 -> 32646 (-2.93%)
helped: 120
HURT: 2
helped stats (abs) min: 1 max: 13 x̄: 8.53 x̃: 13
helped stats (rel) min: 0.30% max: 16.67% x̄: 4.55% x̃: 3.17%
HURT stats (abs)   min: 18 max: 18 x̄: 18.00 x̃: 18
HURT stats (rel)   min: 1.15% max: 2.84% x̄: 2.00% x̃: 2.00%
95% mean confidence interval for instructions value: -9.19 -6.99
95% mean confidence interval for instructions %-change: -5.27% -3.62%
Instructions are helped.

total cycles in shared programs: 411212880 -> 411199636 (<.01%)
cycles in affected programs: 696441 -> 683197 (-1.90%)
helped: 107
HURT: 5
helped stats (abs) min: 2 max: 864 x̄: 124.90 x̃: 146
helped stats (rel) min: 0.03% max: 29.20% x̄: 8.58% x̃: 5.88%
HURT stats (abs)   min: 2 max: 50 x̄: 24.00 x̃: 22
HURT stats (rel)   min: 0.01% max: 5.35% x̄: 1.29% x̃: 0.25%
95% mean confidence interval for cycles value: -136.96 -99.54
95% mean confidence interval for cycles %-change: -9.75% -6.53%
Cycles are helped.

total spills in shared programs: 78623 -> 78631 (0.01%)
spills in affected programs: 66 -> 74 (12.12%)
helped: 0
HURT: 2

total fills in shared programs: 80104 -> 80108 (<.01%)
fills in affected programs: 133 -> 137 (3.01%)
helped: 0
HURT: 2

No changes on Sandy Bridge, Iron Lake, or GM45.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Thomas Helland <thomashelland90@gmail.com>