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>

src/compiler/nir/nir.c

@@ -1489,6 +1489,7 @@ nir_ssa_def_init(nir_instr *instr, nir_ssa_def *def,
                  unsigned bit_size, const char *name)
 {
    def->name = ralloc_strdup(instr, name);
+   def->live_index = UINT_MAX; /* Something clearly OOB */
    def->parent_instr = instr;
    list_inithead(&def->uses);
    list_inithead(&def->if_uses);