i965: make use of nir linking

i965: make use of nir linking

For now linking is just removing unused varyings between stages.

shader-db results BDW:

total instructions in shared programs: 13198288 -> 13191693 (-0.05%)
instructions in affected programs: 48325 -> 41730 (-13.65%)
helped: 473
HURT: 0

total cycles in shared programs: 541184926 -> 541159260 (-0.00%)
cycles in affected programs: 213238 -> 187572 (-12.04%)
helped: 435
HURT: 8

V2:
- lower indirects on demoted inputs as well as outputs.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>