message: Make code walk parallel
  Summary: Faster is better.
diff: @@ -628,7 +628,7 @@ class OptimizeEnums {
  const GeneratedSwitchCases& generated_switch_cases) {
  namespace cp = constant_propagation;
  - walk::code(m_scope, [&](DexMethod*, IRCode& code) {
  + walk::parallel::code(m_scope, [&](DexMethod*, IRCode& code) {
  cfg::ScopedCFG cfg(&code);
  cfg->calculate_exit_block();
  @@ -909,13 +909,13 @@ class OptimizeEnums {
  size_t num_enum_classes{0};
  size_t num_enum_objs{0};
  size_t num_int_objs{0};
  - size_t num_switch_equiv_finder_failures{0};
  + std::atomic<size_t> num_switch_equiv_finder_failures{0};
  size_t num_candidate_generated_methods{0};
  size_t num_removed_generated_methods{0};
  };
  Stats m_stats;
  - std::unordered_set<DexField*> m_lookup_tables_replaced;
  + ConcurrentSet<DexField*> m_lookup_tables_replaced;
  const DexMethod* m_java_enum_ctor;
  const ProguardMap& m_pg_map;
  };
  