nir/builder: Add a build_deref_array_imm helper

nir/builder: Add a build_deref_array_imm helper

Unlike most of the cases in which we do this by hand, the new helper
properly handles non-32-bit pointers.

Reviewed-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>