Auto merge of #111447 - scottmcm:remove-more-assumes, r=thomcc

Remove useless `assume`s from `slice::iter(_mut)`

You were right in rust-lang/rust#111395 (comment),
r? `@the8472`

LLVM already removes these assumes while optimizing, as can be seen in <https://rust.godbolt.org/z/KTfWKbdEM>.

Author: bors