nir: Don't condition 'a-b < 0' -> 'a < b' on is_not_used_by_conditional

nir: Don't condition 'a-b < 0' -> 'a < b' on is_not_used_by_conditional

Now that i965 recognizes that a-b generates the same conditions as 'a <
b', there is no reason to condition this transformation on 'is not used
by conditional.'

Since this was the only user of the is_not_used_by_conditional function,
delete it.

All Gen6+ platforms had similar results. (Skylake shown)
total instructions in shared programs: 14400775 -> 14400595 (<.01%)
instructions in affected programs: 36712 -> 36532 (-0.49%)
helped: 182
HURT: 26
helped stats (abs) min: 1 max: 2 x̄: 1.13 x̃: 1
helped stats (rel) min: 0.15% max: 1.82% x̄: 0.70% x̃: 0.62%
HURT stats (abs)   min: 1 max: 1 x̄: 1.00 x̃: 1
HURT stats (rel)   min: 0.24% max: 1.02% x̄: 0.82% x̃: 0.90%
95% mean confidence interval for instructions value: -0.97 -0.76
95% mean confidence interval for instructions %-change: -0.59% -0.43%
Instructions are helped.

total cycles in shared programs: 532929592 -> 532926345 (<.01%)
cycles in affected programs: 478660 -> 475413 (-0.68%)
helped: 187
HURT: 22
helped stats (abs) min: 2 max: 200 x̄: 20.99 x̃: 18
helped stats (rel) min: 0.23% max: 24.10% x̄: 1.48% x̃: 1.03%
HURT stats (abs)   min: 1 max: 214 x̄: 30.86 x̃: 11
HURT stats (rel)   min: 0.01% max: 23.06% x̄: 3.12% x̃: 0.86%
95% mean confidence interval for cycles value: -19.50 -11.57
95% mean confidence interval for cycles %-change: -1.42% -0.58%
Cycles are helped.

GM45 and Iron Lake had similar results. (Iron Lake shown)
total cycles in shared programs: 177851578 -> 177851810 (<.01%)
cycles in affected programs: 24408 -> 24640 (0.95%)
helped: 2
HURT: 4
helped stats (abs) min: 4 max: 4 x̄: 4.00 x̃: 4
helped stats (rel) min: 0.42% max: 0.47% x̄: 0.44% x̃: 0.44%
HURT stats (abs)   min: 24 max: 108 x̄: 60.00 x̃: 54
HURT stats (rel)   min: 0.52% max: 1.62% x̄: 1.04% x̃: 1.02%
95% mean confidence interval for cycles value: -7.75 85.08
95% mean confidence interval for cycles %-change: -0.39% 1.49%
Inconclusive result (value mean confidence interval includes 0).

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>