anv,i965: Move CS shared lowering into anv

anv,i965: Move CS shared lowering into anv

Right now, OpenGL uses the GLSL lowering for shared variables and anv
uses NIR to lower them.  For a long time, we've done this weird thing
where we do the NIR lowering unconditionally and then add the SLM sizes
from the two together.  This works because one of them will always be 0
but it's a bit sketchy.  Let's just move the NIR-based lowering into
anv_pipeline and get rid of the sketch.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>