nir: Account for atomics in copy propagation.

Otherwise writes get propagated across atomics if no barrier is
used. Without barrier writes should still be visible in the same
invocation, so an atomic has to be considered a write.

CC: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Fixes: b3c61469 "nir: Copy propagation between blocks"
Fixes: 62332d13 "nir: Add a local variable-based copy propagation pass"
(cherry picked from commit 8424cd8f)