[turbofan] Use enum instead of bool for parameter.

TEST=compiler-unittests
R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/517123004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23509 ce2b1a6d-e550-0410-aec6-3dcde31c8c00