Add ToBoolean-conversion of constants in Crankshaft and use it when generating a branch based on a constant.
Review URL: http://codereview.chromium.org/6801050

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7552 ce2b1a6d-e550-0410-aec6-3dcde31c8c00