Different codegen between index and iterator

They should produce the same codegen.
https://godbolt.org/z/W5hKTedd3
```rust
#[inline(never)]
pub fn f_index(data: &mut [u32]) {
    for i in 0..data.len() {
        data[i] += 1;
    }
}
```
```rust
#[inline(never)]
pub fn f_iter(data: &mut [u32]) {
    for v in data.iter_mut() {
        *v += 1;
    }
}
```