The gc should be able to traverse all AllocationSites for decision making. The sites are threaded into a weak list. Special problems include:

* Allocations of AllocationSites occur in generated code, so generated code needs to be able to add to the list. For now I have a special hydrogen instruction, though it would be nice to use general purpose instructions.
* The snapshot contains AllocationSites, and these need to be re-threaded into the list on deserialization.

Something nice is that the AllocationSites are only created in old space, so a special new space visitor isn't required.

BUG=
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/18173013

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@15715 ce2b1a6d-e550-0410-aec6-3dcde31c8c00