intel/nir: Use nir_shrink_vec_array_vars

intel/nir: Use nir_shrink_vec_array_vars

Shader-db results on Kaby Lake:

    total instructions in shared programs: 15177605 -> 15176765 (<.01%)
    instructions in affected programs: 4259 -> 3419 (-19.72%)
    helped: 1
    HURT: 0

    total spills in shared programs: 10954 -> 10855 (-0.90%)
    spills in affected programs: 295 -> 196 (-33.56%)
    helped: 1
    HURT: 0

    total fills in shared programs: 22222 -> 22117 (-0.47%)
    fills in affected programs: 417 -> 312 (-25.18%)
    helped: 1
    HURT: 0

The helped shader is from the OglCSDof synmark test.  On my Kaby Lake
laptop, the actual framerate of the benchmark didn't appear to improve
beyond the noise.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>