nir: Add a helper for creating a "bare" nir_function_impl

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

src/compiler/nir/nir.h

@@ -1732,6 +1732,8 @@ nir_variable *nir_local_variable_create(nir_function_impl *impl,
 nir_function *nir_function_create(nir_shader *shader, const char *name);
 
 nir_function_impl *nir_function_impl_create(nir_function *func);
+/** creates a function_impl that isn't tied to any particular function */
+nir_function_impl *nir_function_impl_create_bare(nir_shader *shader);
 
 nir_block *nir_block_create(nir_shader *shader);
 nir_if *nir_if_create(nir_shader *shader);