nir: Initialize nir_ssa_def::live_index

Previously, this was left uninitialized.  Let's initialize it to an
obviously bogus value so we notice if anyone ever tries to use stale
liveness data.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6210>