nir/lower_subgroups: Rename lower_shuffle to lower_relative_shuffle

This option only applies to relative shuffles (up/down/xor), and in a
moment we're going to add an option to lower normal shuffles, so rename
it.

While we're here, rename lower_shuffle() to lower_to_shuffle() for
similar reasons.

Reviewed-by: Danylo Piliaiev <dpiliaiev@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14412>

src/intel/compiler/brw_nir.c

@@ -861,7 +861,7 @@ brw_preprocess_nir(const struct brw_compiler *compiler, nir_shader *nir,
       .ballot_components = 1,
       .lower_to_scalar = true,
       .lower_vote_trivial = !is_scalar,
-      .lower_shuffle = true,
+      .lower_relative_shuffle = true,
       .lower_quad_broadcast_dynamic = true,
       .lower_elect = true,
    };