Revert "Bug 39129: Speeding up partition_point/lower_bound/upper_bound/ by...

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

This reverts r345525. I'm reverting because that patch apparently caused
a regression on certain platforms (see https://reviews.llvm.org/D53994).
Since we don't fully understand the reasons for the regression, I'm
reverting until we can provide a fix we understand.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@345893 91177308-0d34-0410-b5e6-96231b3b80d8