Summary:
* The atomic load was being optimized out of the benchmark.
* The initializer wasn't being called for the test tags, so `AccessSpreader<ThreadLocalTag>::current()` and `AccessSpreader<PthreadSelfTag>::current()` were always returning 0.

Reviewed By: ot

Differential Revision: D10154155

fbshipit-source-id: 76be9865c9ee591d24aedc64e6faf4fd24350282