intel/compiler: Use gl_shader_stage_uses_workgroup() helpers

Instead of checking for MESA_SHADER_COMPUTE (and KERNEL).  Where
appropriate, also use gl_shader_stage_is_compute().

This allows most of the workgroup-related lowering to be applied to
Task and Mesh shaders.  These will be added later and "inherit" from
cs_prog_data structure.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13629>

src/intel/compiler/brw_nir.c

@@ -1285,7 +1285,7 @@ get_subgroup_size(gl_shader_stage stage,
    case BRW_SUBGROUP_SIZE_REQUIRE_8:
    case BRW_SUBGROUP_SIZE_REQUIRE_16:
    case BRW_SUBGROUP_SIZE_REQUIRE_32:
-      assert(stage == MESA_SHADER_COMPUTE);
+      assert(gl_shader_stage_uses_workgroup(stage));
       /* These enum values are expressly chosen to be equal to the subgroup
        * size that they require.
        */