Also eliminate a useless copy of the map set in case of
HCheckMaps.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/256593011

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@21033 ce2b1a6d-e550-0410-aec6-3dcde31c8c00