intel/nir: Add a memory barrier before barrier()

Our barrier instruction does not implicitly do a memory fence but the
GLSL barrier() intrinsic is supposed to.  The easiest back-portable
solution is to just add the NIR barriers.  We'll sort this out more
properly in later commits.

Cc: mesa-stable@lists.freedesktop.org
Closes: #2138
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
(cherry picked from commit 803fad43)