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>

src/compiler/nir/nir.h

@@ -2661,6 +2661,7 @@ int nir_gs_count_vertices(const nir_shader *shader);
 
 bool nir_split_var_copies(nir_shader *shader);
 bool nir_split_per_member_structs(nir_shader *shader);
+bool nir_split_struct_vars(nir_shader *shader, nir_variable_mode modes);
 
 bool nir_lower_returns_impl(nir_function_impl *impl);
 bool nir_lower_returns(nir_shader *shader);