i965: Use NIR by default for vertex shaders on GEN8+

i965: Use NIR by default for vertex shaders on GEN8+

GLSL IR vs. NIR shader-db results for SIMD8 vertex shaders on Broadwell:

   total instructions in shared programs: 2742062 -> 2681339 (-2.21%)
   instructions in affected programs:     1514770 -> 1454047 (-4.01%)
   helped:                                5813
   HURT:                                  1120

The gained programs are ARB vertext programs that were previously going
through the vec4 backend.  Now that we have prog_to_nir, ARB vertex
programs can go through the scalar backend so they show up as "gained" in
the shader-db results.

Acked-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Acked-by: Matt Turner <mattst88@gmail.com>