Always visit branches during HGraph building

even if constant values indicate that they are unreachable.

BUG=chromium:280333
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/23623009

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16431 ce2b1a6d-e550-0410-aec6-3dcde31c8c00