Auto merge of #148157 - nnethercote:undo-chain, r=saethlin

Adjust successor iterators.

Because rust-lang/rust#148054 was a slight perf regression.

The problem was seemingly because this iterator structure:
```
slice_iter.chain(Option_iter.chain(Option_iter))
```
changed to this:
```
slice_iter.chain(Option_iter).chain(Option_iter)
```
The commit also tweaks the `slice_iter` part, changing `into_iter` to `iter` and using `[]` instead of `(&[])`, for conciseness and consistency.

r? `@saethlin`

Author: bors