The backward propagation was different in the lowering phase versus the propagation phase, leading to some missing truncations.

R=bmeurer@chromium.org
BUG=

Review URL: https://codereview.chromium.org/638533002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24434 ce2b1a6d-e550-0410-aec6-3dcde31c8c00