// types. If the returned resource traces back to an input argument for the
  // SPC, then replace uses of the returned copy with the original input.
  //
  // Note: This does not descend through nested SCPs.
  auto ComesFromBlockArgNumber = [](Value val) -> int {
    while (true) {
      if (auto block_arg = llvm::dyn_cast<BlockArgument>(val)) {
        return block_arg.getArgNumber();
      }
      if (auto identity_op =