Rollup merge of #47333 - arthurprs:iter-position-bounds-check, r=dtolnay

Optimize slice.{r}position result bounds check

Second attempt of https://github.com/rust-lang/rust/pull/45501
Fixes https://github.com/rust-lang/rust/issues/45964

Demo: https://godbolt.org/g/N4mBHp

diff --cc src/libcore/slice/mod.rs
Simple merge

diff --cc src/libcore/tests/slice.rs
Simple merge