i965/nir/fs: Implement new barrier functions for compute shaders

i965/nir/fs: Implement new barrier functions for compute shaders

For these nir intrinsics, we emit the same code as
nir_intrinsic_memory_barrier:

 * nir_intrinsic_memory_barrier_atomic_counter
 * nir_intrinsic_memory_barrier_buffer
 * nir_intrinsic_memory_barrier_image

We treat these nir intrinsics as no-ops:
 * nir_intrinsic_group_memory_barrier
 * nir_intrinsic_memory_barrier_shared

v3:
 * Add comment for no-op cases (curro)

v4:
 * Moving comment to a separate patch authored by curro

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>