message: some micro-optimizations
  Summary:
  This makes it faster.
  This is a behavior-preserving change.
diff: @@ -439,7 +439,7 @@ void Allocator::simplify(interference::Graph* ig,
  // of them here since some of them can have zero weight.
  std::set<reg_t> low;
  // Nodes that may not be colorable
  - std::set<reg_t> high;
  + std::unordered_set<reg_t> high;
  for (const auto& pair : ig->active_nodes()) {
  auto reg = pair.first;
  @@ -504,12 +504,18 @@ void Allocator::simplify(interference::Graph* ig,
  std::min_element(high.begin(), high.end(), [ig](reg_t a, reg_t b) {
  auto& node_a = ig->get_node(a);
  auto& node_b = ig->get_node(b);
  - if (node_a.is_spilt() == node_b.is_spilt()) {
  - // Note that a / b < c / d <=> a * d < c * b.
  - return node_a.spill_cost() * node_b.weight() <
  - node_b.spill_cost() * node_a.weight();
  - }
  + if (node_a.is_spilt() != node_b.is_spilt()) {
  return !node_a.is_spilt() && node_b.is_spilt();
  + }
  + // Note that a / b < c / d <=> a * d < c * b.
  + auto a_value = node_a.spill_cost() * node_b.weight();
  + auto b_value = node_b.spill_cost() * node_a.weight();
  + if (a_value != b_value) {
  + return a_value < b_value;
  + }
  + // Among equivalent options, prefer lowest register number as tie
  + // breaker.
  + return a < b;
  });
  TRACE(REG, 6, "Potentially spilling %u", *spill_candidate_it);
  // Our spill candidate has too many neighbors for us to be certain that we
  @@ -574,7 +580,7 @@ void Allocator::select_ranges(const cfg::ControlFlowGraph& cfg,
  VirtualRegistersFile vreg_file;
  auto src = insn->src(i);
  mark_adjacent(ig, src, reg_transform->map, &vreg_file);
  - vreg_files.emplace(src, vreg_file);
  + vreg_files.emplace(src, std::move(vreg_file));
  }
  vreg_t range_base =
  @@ -604,7 +610,7 @@ void Allocator::select_params(const cfg::ControlFlowGraph& cfg,
  param_regs.emplace_back(dest);
  VirtualRegistersFile vreg_file;
  mark_adjacent(ig, dest, reg_transform->map, &vreg_file);
  - vreg_files.emplace(dest, vreg_file);
  + vreg_files.emplace(dest, std::move(vreg_file));
  }
  auto min_param_reg =
  