llvmpipe: advertise support for subgroups in all stages

Works on CL and also in use by lavapipe. However it needs the
shader_subgroup_size cap to be set to be actually advertized in OpenGL.

Also support subgroup rotate while at it.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/38015>

src/gallium/auxiliary/gallivm/lp_bld_nir.c

@@ -77,6 +77,7 @@ lp_build_opt_nir(struct nir_shader *nir)
          .lower_subgroup_masks = true,
          .lower_relative_shuffle = true,
          .lower_inverse_ballot = true,
+         .lower_rotate_to_shuffle = true,
       };
       NIR_PASS(progress, nir, nir_lower_subgroups, &subgroups_options);
    } while (progress);

src/gallium/drivers/llvmpipe/lp_screen.c

@@ -391,6 +391,9 @@ llvmpipe_init_screen_caps(struct pipe_screen *screen)
    caps->max_point_size_aa = LP_MAX_POINT_WIDTH; /* arbitrary */
    caps->max_texture_anisotropy = 16.0; /* not actually signficant at this time */
    caps->max_texture_lod_bias = 16.0; /* arbitrary */
+
+   caps->shader_subgroup_supported_stages = BITFIELD_MASK(MESA_SHADER_MESH_STAGES);
+   caps->shader_subgroup_supported_features = PIPE_SHADER_SUBGROUP_FEATURE_MASK;
 }