i965/nir/vec4: Select between new nir_vec4 or current vec4_visitor code-paths

i965/nir/vec4: Select between new nir_vec4 or current vec4_visitor code-paths

The NIR->vec4 pass will be activated if both the following conditions are met:

* INTEL_USE_NIR environment variable is defined and is positive (1 or true)
* The stage is vertex shader (support for geometry shaders and
  ARB_vertex_program will be added later).

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>