This has multiple benefits:
- The optimizations are also performed for the `ranges::` versions of the algorithms
- Code duplication is reduced
- it is simpler to add this optimization for other segmented iterators,
like `ranges::join_view::iterator`
- Algorithm code is removed from `<deque>`
Reviewed By: ldionne, huixie90, #libc
Spies: mstorsjo, sstefan1, EricWF, libcxx-commits, mgorny
Differential Revision: https://reviews.llvm.org/D132505
