This is based on Georg's work on typing arithmetic operations (https://codereview.chromium.org/658743002/).

Instead of weakening to bitset types, we weaken to the closest 2^n
limit if we see that we are re-typing a node with a range type (which
means that the node can be part of a cycle, so we might need
to speed up the fixpoint there).

BUG=
R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/636283009

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24848 ce2b1a6d-e550-0410-aec6-3dcde31c8c00