Shrink WatchpointSet.
<https://webkit.org/b/126694>

Reorder the members of WatchpointSet, shrinking it by 8 bytes.
767 kB progression on Membuster3.

Reviewed by Antti Koivisto.

* bytecode/Watchpoint.h:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@161554 268f45cc-cd09-0410-ab3c-d52691b4dbfc