X64: Fix buggy expression in disassembler.  Fixes Issue 408: http://code.google.com/p/v8/issues/detail?id=408
Review URL: http://codereview.chromium.org/160174

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2544 ce2b1a6d-e550-0410-aec6-3dcde31c8c00