Fix representation of Phis for mutable-heapnumber-in-object-literal properties

BUG=v8:3392
LOG=y
R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/328343004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21850 ce2b1a6d-e550-0410-aec6-3dcde31c8c00