i965/vec4: Rewrite dead code elimination to use live in/out.

i965/vec4: Rewrite dead code elimination to use live in/out.

Improves 359 shaders by >=10%
         114 shaders by >=20%
          91 shaders by >=30%
          82 shaders by >=40%
          22 shaders by >=50%
           4 shaders by >=60%
           2 shaders by >=80%

total instructions in shared programs: 5845346 -> 5822422 (-0.39%)
instructions in affected programs:     364979 -> 342055 (-6.28%)

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