st/glsl_to_nir: add more nir opts to st_nir_opts()

st/glsl_to_nir: add more nir opts to st_nir_opts()

All of the current gallium nir driver use these optimisations but
they do so in their backends. Having these called in the backend
only can cause a number of problems:

- Shader compile times are greater because the opts need to do
  significant passes over all shader variants.
- The shader cache is partially defeated due to the significant
  optimisation passes over variants.
- We might miss out on nir linking optimisation opportunities.

Adding these passes to st_nir_opts() alleviates these problems.
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Eric Anholt <eric@anholt.net>