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_fs.h

@@ -535,6 +535,9 @@ public:
    const unsigned dispatch_width; /**< 8, 16 or 32 */
    unsigned max_dispatch_width;
 
+   /* The API selected subgroup size */
+   unsigned api_subgroup_size; /**< 0, 8, 16, 32 */
+
    struct shader_stats shader_stats;
 
    brw::fs_builder bld;