MIPS: Ensure that all descriptors have a valid enumeration index, and replace NextEnumIndex with LastAdded.

Port r12042 (ce65764e)

Original commit message:
The LastAdded points to the descriptor that was last added to the array. From the descriptor we can deduce the NextEnumerationIndex. This allows us to quickly find the property that we are transitioning to, which is necessary for transition-intensive code, eg JSON parsing.

BUG=
TEST=

Review URL: https://chromiumcodereview.appspot.com/10690178
Patch from Akos Palfi <palfia@homejinni.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12076 ce2b1a6d-e550-0410-aec6-3dcde31c8c00