intel/nir: Re-run int64 lowering in postprocess_nir

intel/nir: Re-run int64 lowering in postprocess_nir

We're about to start doing 64-bit pointer calculations in ANV.  They
will get applied after brw_preprocess_nir which is where we currently do
64-bit integer arithmetic lowering.  Because we're adding 64-bit integer
arithmetic after the initial lowering has happened, we need to lower
again.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>