gallium/ntt: Don't manually reindex instrs.

Getting the live SSA defs will do it if necessary, and that liveness is
what we use the instr index for.  (We used to need to do it manually, and
cleanups for merging resulted in the index being treated as metadata).

Acked-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7658>

src/gallium/auxiliary/nir/nir_to_tgsi.c

@@ -2161,8 +2161,6 @@ ntt_emit_impl(struct ntt_compile *c, nir_function_impl *impl)
    nir_index_ssa_defs(impl);
    nir_index_local_regs(impl);
 
-   nir_index_instrs(impl);
-
    c->impl = impl;
    c->liveness = nir_live_ssa_defs_per_instr(impl);