i965/nir: Use signed integer type for booleans

i965/nir: Use signed integer type for booleans

FS instructions with NIR on i965:
total instructions in shared programs: 2663561 -> 2619051 (-1.67%)
instructions in affected programs:     1612965 -> 1568455 (-2.76%)
helped:                                5455
HURT:                                  12

FS instructions with NIR on g4x:
total instructions in shared programs: 2352633 -> 2307908 (-1.90%)
instructions in affected programs:     1441842 -> 1397117 (-3.10%)
helped:                                5463
HURT:                                  11

FS instructions with NIR on ilk:
total instructions in shared programs: 3997305 -> 3934278 (-1.58%)
instructions in affected programs:     2189409 -> 2126382 (-2.88%)
helped:                                8969
HURT:                                  22

FS instructions with NIR on hsw (snb and ivb were similar):
total instructions in shared programs: 4109389 -> 4109242 (-0.00%)
instructions in affected programs:     109869 -> 109722 (-0.13%)
helped:                                339
HURT:                                  190

No SIMD16 programs were gained or lost on any platform

Reviewed-by: Matt Turner <mattst88@gmail.com>