Folding of bounded dynamic size allocations with const size allocations.

R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/301973014

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21675 ce2b1a6d-e550-0410-aec6-3dcde31c8c00