Merged r16393, r16396 into trunk branch.

Insert allocation memento clear instructions after the dominating allocate instruction.

Merge verbatim descriptors from other (the descriptor of the map being updated) rather than this (descriptors of the most updated map found in the transition tree).

BUG=v8:2863
R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/23706002

git-svn-id: https://v8.googlecode.com/svn/trunk@16400 ce2b1a6d-e550-0410-aec6-3dcde31c8c00