intel/fs: fix subgroup invocation read bounds checking

nir->info.subgroup_size can be set to an enum :
  SUBGROUP_SIZE_VARYING = 0
  SUBGROUP_SIZE_UNIFORM = 1
  SUBGROUP_SIZE_API_CONSTANT = 2
  SUBGROUP_SIZE_FULL_SUBGROUPS = 3

So compute the API subgroup size value and compare it to the dispatch
size to determine whether we need some bound checking.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: 9ac192d79d ("intel/fs: bound subgroup invocation read to dispatch size")
Reviewed-by: Marcin Ślusarz <marcin.slusarz@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21856>

src/intel/compiler/brw_nir.c

@@ -1689,6 +1689,13 @@ get_subgroup_size(const struct shader_info *info, unsigned max_subgroup_size)
    unreachable("Invalid subgroup size type");
 }
 
+unsigned
+brw_nir_api_subgroup_size(const nir_shader *nir,
+                          unsigned hw_subgroup_size)
+{
+   return get_subgroup_size(&nir->info, hw_subgroup_size);
+}
+
 void
 brw_nir_apply_key(nir_shader *nir,
                   const struct brw_compiler *compiler,