Dead shift in ConcurrentHashMapSegment

Dead shift in ConcurrentHashMapSegment

Summary: Original problem detected by compiling with `-Werror,-Wunused-value`. On further inspection the only place which uses this detail class ensures that the `max_size` parameter is a power of two already, so we can discard the logic to manipulate `max_size` and put a `CHECK` clause in its place to guard against future changes to the caller that break this assumption.

Reviewed By: yfeldblum

Differential Revision: D5799110

fbshipit-source-id: d21ed9ff196d54ef91e38254df8b1b88bbf29275