software prefetching for F14Value iteration

Summary:
When moving from chunk i to i-1 in an F14{Value,Node}{Map,Set}
iteration, this diff prefetches chunk i-2.  This code will never be run
for small maps and sets, since they fit in a single chunk, and it does
not affect the fast-path of intra-chunk operator++.

Reviewed By: shixiao

Differential Revision: D8395569

fbshipit-source-id: 60cace5997222fd7d1fa03ffcee93f439f2fcf03

folly/container/detail/F14Table.h

@@ -846,6 +846,7 @@ class F14ItemIter {
       }
       --c;
       auto last = c->lastOccupied();
+      prefetchAddr(&*c - 1);
       if (LIKELY(last.hasIndex())) {
         index_ = last.index();
         itemPtr_ = std::pointer_traits<ItemPtr>::pointer_to(c->item(index_));