nir: Add a reg_intrinsics flag to nir_convert_from_ssa

It doesn't do anything yet. We leave that to the subsequent patches so we can
keep the tree-wide refactor as simple as possible.

Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23089>

src/intel/compiler/brw_nir.c

@@ -1722,7 +1722,7 @@ brw_postprocess_nir(nir_shader *nir, const struct brw_compiler *compiler,
 
    nir_validate_ssa_dominance(nir, "before nir_convert_from_ssa");
 
-   OPT(nir_convert_from_ssa, true);
+   OPT(nir_convert_from_ssa, true, false);
 
    if (!is_scalar) {
       OPT(nir_move_vec_src_uses_to_dest);