st/nir: Call st_glsl_to_nir_post_opts before interface unification

st/nir: Call st_glsl_to_nir_post_opts before interface unification

Since 95415a54c007a909f0fb9d1b2567beecbd73d403 st_finalize_nir_before_variants
calls to nir_shader_gather_info, so finalization should be moved
before interface unification.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/3482
Fixes: 95415a54c007a909f0fb9d1b2567beecbd73d403
Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6647>