Auto merge of #102535 - scottmcm:optimize-split-at-partition-point, r…

…=thomcc

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`