Auto merge of #128250 - Amanieu:select_unpredictable, r=nikic

Add `select_unpredictable` to force LLVM to use CMOV

Since https://reviews.llvm.org/D118118, LLVM will no longer turn CMOVs into branches if it comes from a `select` marked with an `unpredictable` metadata attribute.

This PR introduces `core::intrinsics::select_unpredictable` which emits such a `select` and uses it in the implementation of `binary_search_by`.

Author: bors