nir: fix lower_non_uniform_access pass

Obviously missing the instruction insertion into the SSA list.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: 3bd54576 ("nir: Add a lowering pass for non-uniform resource access")
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
(cherry picked from commit 391a836e)