Fix assignment of function name constant.

If it's shadowed by a variable of the same name and both are forcibly
context-allocated, the function is assigned to the wrong context slot.

R=rossberg@chromium.org
BUG=v8:3138
LOG=Y

Review URL: https://codereview.chromium.org/159903008

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19379 ce2b1a6d-e550-0410-aec6-3dcde31c8c00