folly: improve ProducerConsumerQueueBenchmark (misplaced doNotOptimizeAway)

Summary:
as per discussion from https://phabricator.fb.com/D1274352

After changing the queue length to be a small value (100) I didn't see
improvement from cache alignment of atomics (too much noise). Dropping
that and just fixing the benchmark.

Test Plan: n/a

Reviewed By: ngbronson@fb.com

FB internal diff: D1276591

folly/test/ProducerConsumerQueueBenchmark.cpp

@@ -72,8 +72,8 @@ struct ThroughputTest {
     for (int i = 0; i < iters_; ++i) {
       ThroughputType item = 0;
       while (!queue_.read(item)) {
-        doNotOptimizeAway(item);
       }
+      doNotOptimizeAway(item);
     }
   }