message: RemoveUnusedArgs check for null virtualscope
  Summary: A virtual scope is null for undefined methods; add a check
diff: @@ -199,11 +199,13 @@ RemoveArgs::MethodStats RemoveArgs::update_meths_with_unused_args() {
  }
  // If a method is devirtualizable, proceed with live arg computation.
  - auto virtual_scope = m_type_system.find_virtual_scope(method);
  - if (method->is_virtual() && !is_non_virtual_scope(virtual_scope)) {
  + if (method->is_virtual()) {
  + auto virt_scope = m_type_system.find_virtual_scope(method);
  + if (virt_scope == nullptr || !is_non_virtual_scope(virt_scope)) {
  // TODO: T31388603 -- Remove unused args for true virtuals.
  return method_stats;
  }
  + }
  std::vector<IRInstruction*> dead_insns;
  auto live_arg_idxs = compute_live_args(method, num_args, &dead_insns);
  