nir: Introduce workgroup_index and ability to lower workgroup_id to it.

The workgroup_index is intended for situations when a 3 dimensional
workgroup_id is not available on the HW, but a 1 dimensional index is.
In this case, we can use lower the 3D ID to use this.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15103>

src/compiler/nir/nir_divergence_analysis.c

@@ -264,6 +264,7 @@ visit_intrinsic(nir_shader *shader, nir_intrinsic_instr *instr)
          assert(stage == MESA_SHADER_TESS_CTRL);
       break;
 
+   case nir_intrinsic_load_workgroup_index:
    case nir_intrinsic_load_workgroup_id:
       assert(gl_shader_stage_uses_workgroup(stage));
       if (stage == MESA_SHADER_COMPUTE)