message: fix unused variable
  Summary: I called opcode, but forgot to use it
diff: @@ -176,11 +176,11 @@ class AliasFixpointIterator final
  // we need to make sure the dest and src of check-cast stay identical,
  // because the dest is simply an alias to the src. See the comments in
  // IRInstruction.h for details.
  - insn->opcode() != OPCODE_CHECK_CAST &&
  + op != OPCODE_CHECK_CAST &&
  // The ART verifier checks that monitor-{enter,exit} instructions use
  // the same register:
  // http://androidxref.com/6.0.0_r5/xref/art/runtime/verifier/register_line.h#325
  - !is_monitor(insn->opcode())) {
  + !is_monitor(op)) {
  for (size_t i = 0; i < insn->srcs_size(); ++i) {
  Register r = insn->src(i);
  Register rep = get_rep(r, aliases, get_max_addressable(insn, i));
  