Port optimized comparison of a string to a constant single character string to X64 platform.  Fix small mistake on ia32 platform.
Review URL: http://codereview.chromium.org/1627014

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4398 ce2b1a6d-e550-0410-aec6-3dcde31c8c00