Squeeze the layout of variable proxy nodes.

Around 200MB less peak memory usage in the bug mentioned below.

BUG=417697
LOG=y
R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/629983002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24413 ce2b1a6d-e550-0410-aec6-3dcde31c8c00