MIPS: The gc should be able to traverse all AllocationSites for decision making.

MIPS: The gc should be able to traverse all AllocationSites for decision making.

Port r15715 (2e830d4)

Original commit message:
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=

[kisg, 2013-07-17 18:28:37]

LGTM

[palfia, 2013-07-17 18:40:08]

Committed as r15732.