Fix handling of constant global variable assignments.

BUG=347904
LOG=y
R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/184303003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19594 ce2b1a6d-e550-0410-aec6-3dcde31c8c00