nir: Add nir_foreach_function_impl helper

nir: Add nir_foreach_function_impl helper

Most users of nir_foreach_function actually want the nir_function_impl, not the
nir_function, and want to skip empty functions (though some graphics-specific
passes sometimes fail to do that part). Add a nir_foreach_function_impl macro
to make that case more ergonomic.

   nir_foreach_function_impl(impl, shader) {
      ...
      foo(impl)
   }

is equivalent to:

   nir_foreach_function(func, shader) {
      if (func->impl) {
         ...
         foo(func->impl);
      }
   }

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23807>