message: Don't remap when nothing linked
  Summary: This is a behavior-preserving change.
diff: @@ -363,6 +363,7 @@ bool Allocator::coalesce(interference::Graph* ig, cfg::ControlFlowGraph& cfg) {
  auto ii = InstructionIterable(cfg);
  auto old_coalesce_count = m_stats.moves_coalesced;
  cfg::CFGMutation m(cfg);
  + bool any_linked{false};
  for (auto it = ii.begin(); it != ii.end(); ++it) {
  auto insn = it->insn;
  auto op = insn->opcode();
  @@ -388,6 +389,7 @@ bool Allocator::coalesce(interference::Graph* ig, cfg::ControlFlowGraph& cfg) {
  } else if (ig->is_coalesceable(dest, src)) {
  // This unifies the two trees represented by dest and src
  aliases.link(dest, src);
  + any_linked = true;
  // Since link() doesn't tell us whether dest or src is the root of the
  // newly merged trees, we have to use find_set() to figure that out.
  auto parent = dest;
  @@ -408,11 +410,13 @@ bool Allocator::coalesce(interference::Graph* ig, cfg::ControlFlowGraph& cfg) {
  m.flush();
  + if (any_linked) {
  transform::RegMap reg_map;
  for (reg_t i = 0; i < cfg.get_registers_size(); ++i) {
  reg_map.emplace(i, aliases.find_set(i));
  }
  transform::remap_registers(cfg, reg_map);
  + }
  return m_stats.moves_coalesced != old_coalesce_count;
  }
  