Enable more tests for simplified lowering after assuming signed for word32.

R=mstarzinger@chromium.org
BUG=

Review URL: https://codereview.chromium.org/461933002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23125 ce2b1a6d-e550-0410-aec6-3dcde31c8c00