mesa/compiler: add stage to shader_info

mesa/compiler: add stage to shader_info

This will allow us to simplify the current program logic for SSO.

Also since we aim to detach shader_info from nir_shader this will come
in handy avoiding passing nir_shader around just to keep track of
the stage we are dealing with.

V2: set stage for arb asm programs also.

Reviewed-by: Eric Anholt <eric@anholt.net>