nir: Introduce nir_metadata_instr_index for nir_index_instr() being current.

This will be useful to remove the live_index field from nir_ssa_def.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3395>
3 files changed