More efficient use of space in AllocationSite.

We can eliminate one word from the object by sharing the pretenuring decision and the found memento count.

R=bmeurer@chromium.org, hpayer@chromium.org

Review URL: https://codereview.chromium.org/132063004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18554 ce2b1a6d-e550-0410-aec6-3dcde31c8c00