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/compiler/nir/nir.h

@@ -2178,6 +2178,7 @@ typedef struct nir_shader_compiler_options {
    bool lower_helper_invocation;
 
    bool lower_cs_local_index_from_id;
+   bool lower_cs_local_id_from_index;
 
    bool lower_device_index_to_zero;