Use unsigned type bitsets to limit undefined behaviour

R=bmeurer@chromium.org, svenpanne@chromium.org
BUG=

Review URL: https://codereview.chromium.org/555153003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23916 ce2b1a6d-e550-0410-aec6-3dcde31c8c00