nir: Don't compare b2f or b2i with zero

nir: Don't compare b2f or b2i with zero

All of the shaders that had loops changed were in Tomb Raider.  The one
shader that lost SIMD16 is one of those.

Skylake
total instructions in shared programs: 14391653 -> 14390468 (<.01%)
instructions in affected programs: 111891 -> 110706 (-1.06%)
helped: 501
HURT: 0
helped stats (abs) min: 1 max: 155 x̄: 2.37 x̃: 1
helped stats (rel) min: 0.05% max: 21.54% x̄: 1.61% x̃: 1.01%
95% mean confidence interval for instructions value: -3.23 -1.50
95% mean confidence interval for instructions %-change: -1.77% -1.45%
Instructions are helped.

total cycles in shared programs: 532793024 -> 532776598 (<.01%)
cycles in affected programs: 987682 -> 971256 (-1.66%)
helped: 348
nnHURT: 41
helped stats (abs) min: 1 max: 3074 x̄: 54.91 x̃: 18
helped stats (rel) min: 0.05% max: 32.24% x̄: 3.36% x̃: 1.68%
HURT stats (abs)   min: 1 max: 422 x̄: 65.39 x̃: 24
HURT stats (rel)   min: 0.09% max: 39.29% x̄: 9.50% x̃: 2.02%
95% mean confidence interval for cycles value: -64.08 -20.38
95% mean confidence interval for cycles %-change: -2.78% -1.23%
Cycles are helped.

total loops in shared programs: 4854 -> 4829 (-0.52%)
loops in affected programs: 27 -> 2 (-92.59%)
helped: 18
HURT: 0

LOST:   1
GAINED: 0

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>