Merged r18483, r18484 into 3.23 branch.

Fix selection of popular pages in store buffer.

Correctly handle instances without elements in polymorphic keyed load/store.

BUG=331444,331416
LOG=N
R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/140703002

git-svn-id: https://v8.googlecode.com/svn/branches/3.23@18637 ce2b1a6d-e550-0410-aec6-3dcde31c8c00