intel,nir: Move gl_LocalInvocationID lowering to nir_lower_system_values

It's not at all intel-specific; the formula is dictated by OpenGL and
Vulkan.  The only intel-specific thing is that we need the lowering.  As
a nice side-effect, the new version is variable-group-size ready.

Reviewed-by: Plamena Manolova <plamena.manolova@intel.com>

src/intel/compiler/brw_compiler.c

@@ -42,6 +42,7 @@
    .lower_fdiv = true,                                                        \
    .lower_flrp64 = true,                                                      \
    .lower_ldexp = true,                                                       \
+   .lower_cs_local_id_from_index = true,                                      \
    .lower_device_index_to_zero = true,                                        \
    .native_integers = true,                                                   \
    .use_interpolated_input_intrinsics = true,                                 \