Tell LLVM that `partition_point` returns a valid fencepost This was already done for a successful `binary_search`, but this way `partition_point` can get similar optimizations. Demonstration that nightly can't do this optimization today, and leaves in the panicking path: <https://play.rust-lang.org/?version=nightly&mode=release&edition=2021&gist=e1074cd2faf5f68e49cffd728ded243a> r? `@thomcc`