Handle Symbols in the LBranch instruction on x64.

This was an omission in https://code.google.com/p/v8/source/detail?r=14051.

R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/22578002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16101 ce2b1a6d-e550-0410-aec6-3dcde31c8c00