spirv: don't discard access set by vtn_pointer_dereference

We can have a access flag already set here so just augment the
existing ones.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: 0fb61dfd ("spirv: propagate access qualifiers through ssa & pointer")
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
(cherry picked from commit 7deb5ec0)