nir: Simplify i2b with negated or abs operand

nir: Simplify i2b with negated or abs operand

This enables removing ssa_201 and ssa_202 in sequences like:

                 vec1 ssa_200 = flt ssa_199, ssa_194
                 vec1 ssa_201 = b2i ssa_200
                 vec1 ssa_202 = i2b -ssa_201

shader-db results:

Sandy Bridge
total instructions in shared programs: 8462257 -> 8462180 (-0.00%)
instructions in affected programs: 3846 -> 3769 (-2.00%)
helped: 35
HURT: 0

total cycles in shared programs: 117542934 -> 117542462 (-0.00%)
cycles in affected programs: 20072 -> 19600 (-2.35%)
helped: 20
HURT: 1

Ivy Bridge
total instructions in shared programs: 7775252 -> 7775137 (-0.00%)
instructions in affected programs: 3645 -> 3530 (-3.16%)
helped: 35
HURT: 0

total cycles in shared programs: 65760522 -> 65760068 (-0.00%)
cycles in affected programs: 21082 -> 20628 (-2.15%)
helped: 25
HURT: 2

Haswell
total instructions in shared programs: 7108666 -> 7108589 (-0.00%)
instructions in affected programs: 3253 -> 3176 (-2.37%)
helped: 35
HURT: 0

total cycles in shared programs: 64675726 -> 64675272 (-0.00%)
cycles in affected programs: 21034 -> 20580 (-2.16%)
helped: 26
HURT: 1

Broadwell / Skylake
total instructions in shared programs: 8980912 -> 8980835 (-0.00%)
instructions in affected programs: 3223 -> 3146 (-2.39%)
helped: 35
HURT: 0

total cycles in shared programs: 70077926 -> 70077904 (-0.00%)
cycles in affected programs: 21886 -> 21864 (-0.10%)
helped: 21
HURT: 6

G45 and Ironlake showed no change.

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