nir: Don't abs the result of b2f or b2i

nir: Don't abs the result of b2f or b2i

In the results below, 2 SIMD16 shaders in Trine are lost.

G4X
total instructions in shared programs: 4012279 -> 4011108 (-0.03%)
instructions in affected programs: 116776 -> 115605 (-1.00%)
helped: 339
HURT: 0

total cycles in shared programs: 84315862 -> 84313584 (-0.00%)
cycles in affected programs: 1767232 -> 1764954 (-0.13%)
helped: 274
HURT: 81

Ironlake
total instructions in shared programs: 6399073 -> 6396998 (-0.03%)
instructions in affected programs: 218050 -> 215975 (-0.95%)
helped: 600
HURT: 0

total cycles in shared programs: 128892088 -> 128888810 (-0.00%)
cycles in affected programs: 2867452 -> 2864174 (-0.11%)
helped: 422
HURT: 137

Sandy Bridge
total instructions in shared programs: 8462174 -> 8460759 (-0.02%)
instructions in affected programs: 178529 -> 177114 (-0.79%)
helped: 596
HURT: 0

total cycles in shared programs: 117542276 -> 117534098 (-0.01%)
cycles in affected programs: 1239166 -> 1230988 (-0.66%)
helped: 369
HURT: 150

Ivy Bridge
total instructions in shared programs: 7775131 -> 7773410 (-0.02%)
instructions in affected programs: 162903 -> 161182 (-1.06%)
helped: 590
HURT: 0

total cycles in shared programs: 65759882 -> 65747268 (-0.02%)
cycles in affected programs: 1004354 -> 991740 (-1.26%)
helped: 467
HURT: 141

Haswell
total instructions in shared programs: 7107786 -> 7106327 (-0.02%)
instructions in affected programs: 140954 -> 139495 (-1.04%)
helped: 590
HURT: 0

total cycles in shared programs: 64668028 -> 64655322 (-0.02%)
cycles in affected programs: 967080 -> 954374 (-1.31%)
helped: 452
HURT: 149

LOST:   2
GAINED: 0

Broadwell
total instructions in shared programs: 8980029 -> 8978287 (-0.02%)
instructions in affected programs: 197232 -> 195490 (-0.88%)
helped: 715
HURT: 0

total cycles in shared programs: 70070448 -> 70055970 (-0.02%)
cycles in affected programs: 975724 -> 961246 (-1.48%)
helped: 471
HURT: 111

LOST:   2
GAINED: 0

Skylake
total instructions in shared programs: 9115178 -> 9113436 (-0.02%)
instructions in affected programs: 203012 -> 201270 (-0.86%)
helped: 715
HURT: 0

total cycles in shared programs: 68848660 -> 68834004 (-0.02%)
cycles in affected programs: 993888 -> 979232 (-1.47%)
helped: 473
HURT: 116

LOST:   2
GAINED: 0

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