spirv: Use nir_const_value for spec constants

spirv: Use nir_const_value for spec constants

When we originally wrote spirv_to_nir we didn't have a good scalar value
union to handily use so we rolled our own thing for spec constants.  Now
that we have nir_const_value, we can use that and simplify a bunch of
the spec constant logic.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Acked-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4675>