nir/algebraic: Simplify comparison with sequential integers starting with 0

nir/algebraic: Simplify comparison with sequential integers starting with 0

All of the affected shaders are Unreal4 demos.

All Gen6+ platforms had similar results. (Skylake shown)
total instructions in shared programs: 15437170 -> 15437001 (<.01%)
instructions in affected programs: 21536 -> 21367 (-0.78%)
helped: 43
HURT: 0
helped stats (abs) min: 1 max: 4 x̄: 3.93 x̃: 4
helped stats (rel) min: 0.68% max: 1.01% x̄: 0.80% x̃: 0.80%
95% mean confidence interval for instructions value: -4.07 -3.79
95% mean confidence interval for instructions %-change: -0.83% -0.77%
Instructions are helped.

total cycles in shared programs: 383007896 -> 383007378 (<.01%)
cycles in affected programs: 158640 -> 158122 (-0.33%)
helped: 38
HURT: 4
helped stats (abs) min: 1 max: 48 x̄: 13.89 x̃: 6
helped stats (rel) min: 0.03% max: 1.01% x̄: 0.33% x̃: 0.19%
HURT stats (abs)   min: 2 max: 3 x̄: 2.50 x̃: 2
HURT stats (rel)   min: 0.06% max: 0.09% x̄: 0.08% x̃: 0.08%
95% mean confidence interval for cycles value: -16.90 -7.77
95% mean confidence interval for cycles %-change: -0.39% -0.19%
Cycles are helped.

Iron Lake and GM45 had similar results. (Iron Lake shown)
total instructions in shared programs: 8213746 -> 8213745 (<.01%)
instructions in affected programs: 127 -> 126 (-0.79%)
helped: 1
HURT: 0

total cycles in shared programs: 187734146 -> 187734144 (<.01%)
cycles in affected programs: 2132 -> 2130 (-0.09%)
helped: 1
HURT: 0

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