Bug 39129: Speeding up partition_point/lower_bound/upper_bound/ by using...

Bug 39129: Speeding up partition_point/lower_bound/upper_bound/ by using unsigned division by 2 when possible.

Patch by Denis Yaroshevskiy (denis.yaroshevskij@gmail.com)

The rational and measurements can be found in the bug description: https://bugs.llvm.org/show_bug.cgi?id=39129

Reviewed as https://reviews.llvm.org/D52697


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@345525 91177308-0d34-0410-b5e6-96231b3b80d8