intel/fs: Ignore SLM fences if shared is unused

intel/fs: Ignore SLM fences if shared is unused

Found this nugget while looking at the ACO driver.  It seems sensible to
avoid SLM fences if there is no SLM.  This also makes the check depend
on SLM usage rather than just shader stage which will be useful if we
ever implement task/mesh because task shaders also have SLM.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Reviewed-by: Sagar Ghuge <sagar.ghuge@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13092>