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