st/mesa: Optionally call nir_vectorize_tess_levels()

This lets us vectorize gl_TessLevel{Inner,Outer} writes, using a pass
developed for RADV.  Not all backends are prepared to handle this, so
we make it optional.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Caio Oliveira <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17944>

src/compiler/nir/nir.h

@@ -3495,6 +3495,7 @@ typedef struct nir_shader_compiler_options {
     * for IO purposes and would prefer loads/stores be vectorized.
     */
    bool vectorize_io;
+   bool vectorize_tess_levels;
    bool lower_to_scalar;
    nir_instr_filter_cb lower_to_scalar_filter;