i965: Move postprocess_nir to codegen time

i965: Move postprocess_nir to codegen time

This allows us to insert NIR passes between initial NIR compilation and
optimization (link time) and actual backend code-gen.  In particular, it
will allow us to do shader variants in NIR and share some of that shader
variant code between backends.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>