Vec<u8> clone in rustc 1.33.0 is 3 times slower than rustc 1.29.0

Benchmark code:

```rust
#[bench]
fn bench(b: &mut test::Bencher) {
    let raw = vec![0u8; 1000];
    b.iter(|| {
        test::black_box(test::black_box(&raw).clone());
    });
}
```

In rustc 1.29.0-nightly (4f3c7a472 2018-07-17): 32 ns/iter (+/- 34)
In rustc 1.33.0-nightly (9eac38634 2018-12-31): 127 ns/iter (+/- 45)
