Slice patterns don't work well with ref mut

Minimal example: https://is.gd/40m1am

```rust
#![feature(slice_patterns)]
fn main() {
    let mut buf = [0, 1];
    let [ref mut a, ref mut b] = buf;
}
```

This will complain about multiple mutable borrows, even though it should be valid.