compiler/nir: make lowering global-id to local-id optional

For D3D12, we don't want to lower this, as there's a dedicated global-id
system-value that might be faster to use, depending on the hardware.

Reviewed-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5891>
2 files changed