nir: Drop nir_alu_dest

Instead, we replace it directly with nir_def.  We could replace it with
nir_dest but the next commit gets rid of that so this avoids unnecessary
churn.  Most of this commit was generated by sed:

   sed -i -e 's/dest.dest.ssa/def/g' src/**/*.h src/**/*.c src/**/*.cpp

There were a few manual fixups required in the nir_legacy.c and
nir_from_ssa.c as nir_legacy_reg and nir_parallel_copy_entry both have a
similar pattern.

Acked-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24674>

src/compiler/nir/nir_opt_dce.c

@@ -60,7 +60,7 @@ is_live(BITSET_WORD *defs_live, nir_instr *instr)
       return true;
    case nir_instr_type_alu: {
       nir_alu_instr *alu = nir_instr_as_alu(instr);
-      return is_def_live(&alu->dest.dest.ssa, defs_live);
+      return is_def_live(&alu->def, defs_live);
    }
    case nir_instr_type_deref: {
       nir_deref_instr *deref = nir_instr_as_deref(instr);