Use is_constructible instead of is_convertible.

Before this, all objects would follow the slow path (since nothing is
convertible to AlphaNum since it has a private copy constructor).

Before:
entry {
  name: "MicroBenchmarks.benchmark_defun_matmul_2_by_2_CPU"
  iters: 30000
  wall_time: 67.5895690918
  extras {
    key: "examples_per_sec"
    value {
      double_value: 14795.1823549
    }
  }
}

After:
entry {
  name: "MicroBenchmarks.benchmark_defun_matmul_2_by_2_CPU"
  iters: 30000
  wall_time: 61.0044002533
  extras {
    key: "examples_per_sec"
    value {
      double_value: 16392.2601623
    }
  }
}
PiperOrigin-RevId: 193247183