Fix manual allocation folding of RegExpConstructResult.

R=mstarzinger@chromium.org
BUG=409533
LOG=N

Review URL: https://codereview.chromium.org/532453003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23543 ce2b1a6d-e550-0410-aec6-3dcde31c8c00