spirv: don't split memory barriers

spirv: don't split memory barriers

If the SPIR-V had a shared+image memory barrier, we would emit two NIR
barriers: a shared barrier and an image barrier.

Unlike a single barrier, two barriers allows transformations such as:

intrinsic image_deref_store (ssa_27, ssa_33, ssa_34, ssa_32, ssa_25) (1)
intrinsic memory_barrier_shared () ()
intrinsic memory_barrier_image () ()
intrinsic store_shared (ssa_35, ssa_24) (0, 1, 4, 0)
->
intrinsic memory_barrier_shared () ()
intrinsic store_shared (ssa_35, ssa_24) (0, 1, 4, 0)
intrinsic image_deref_store (ssa_27, ssa_33, ssa_34, ssa_32, ssa_25) (1)
intrinsic memory_barrier_image () ()

This commit fixes two dEQP-VK.memory_model.* CTS tests with ACO.

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5951>