Auto merge of #137795 - Jarcho:idx_opt, r=davidtwco

Allow bounds checks when enumerating `IndexSlice` to be elided

Without this hint, each loop iteration has to separately bounds check the index. See https://godbolt.org/z/zrfPY4Ten for an example.

This is technically a behaviour change, but only in cases where the compiler is going to crash anyways.

Author: bors