nir: Add a structure splitting pass

nir: Add a structure splitting pass

This pass doesn't really do much now because nir_lower_vars_to_ssa can
already see through structures and considers them to be "split".  This
pass exists to help other passes more easily see through structure
variables.  If a back-end does implement arrays using scratch or
indirects on registers, having more smaller arrays is likely to have
better memory efficiency.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>