message: remove `seen` set
  Summary: Dead instructions are unique by construction. Apparently, this didn't use the be the case at some point in ancient Redex history where cfg edges were represented in a less straightforward way, but that time is long gone.
diff: @@ -150,7 +150,6 @@ void LocalDce::dce(cfg::ControlFlowGraph& cfg,
  get_dead_instructions(cfg, blocks, &any_init_class_insns);
  // Remove dead instructions.
  - std::unordered_set<IRInstruction*> seen;
  cfg::CFGMutation mutation(cfg);
  std::unique_ptr<npe::NullPointerExceptionCreator> npe_creator;
  size_t npe_instructions = 0;
  @@ -159,9 +158,6 @@ void LocalDce::dce(cfg::ControlFlowGraph& cfg,
  cfg::Block* b = pair.first;
  const IRList::iterator& it = pair.second;
  auto insn = it->insn;
  - if (!seen.emplace(insn).second) {
  - continue;
  - }
  auto cfg_it = b->to_cfg_instruction_iterator(it);
  DexMethod* method;
  if (m_may_allocate_registers && m_method_override_graph &&
  