Remove uses of CanBeNegative() in HMod.

BUG=v8:3204
LOG=y
R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/195793016

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19878 ce2b1a6d-e550-0410-aec6-3dcde31c8c00