i965: use nir loop unrolling pass

i965: use nir loop unrolling pass

shader-db results for BDW:

total instructions in shared programs: 12589614 -> 12590119 (0.00%)
instructions in affected programs: 50525 -> 51030 (1.00%)
helped: 7
HURT: 145

total cycles in shared programs: 241524604 -> 241490502 (-0.01%)
cycles in affected programs: 1941404 -> 1907302 (-1.76%)
helped: 302
HURT: 449

total loops in shared programs: 4245 -> 2947 (-30.58%)
loops in affected programs: 1535 -> 237 (-84.56%)
helped: 1142
HURT: 0

total spills in shared programs: 14453 -> 14453 (0.00%)
spills in affected programs: 0 -> 0
helped: 0
HURT: 0

total fills in shared programs: 18984 -> 18984 (0.00%)
fills in affected programs: 0 -> 0
helped: 0
HURT: 0

LOST:   26
GAINED: 15

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