Implement `pick[23]_mut` via `get_disjoint_mut` [rustc cleanup]

[scottmcm]

The IndexSlice internal type has two methods

• https://doc.rust-lang.org/nightly/nightly-rustc/rustc_index/struct.IndexSlice.html#method.pick2_mut
• https://doc.rust-lang.org/nightly/nightly-rustc/rustc_index/struct.IndexSlice.html#method.pick3_mut

that offer a safe interface to getting multiple mutable borrows into the slice.

Now that https://doc.rust-lang.org/beta/std/primitive.slice.html#method.get_disjoint_mut exists, their current non-trivial implementations can be replaced with a call to that stabilized method (and some pattern-matching and unwrapping).

It's probably not worth changing all the code to stop using pick[23]_mut; just swapping out the implementation is enough.

[Eclips4]

@rustbot claim