[turbofan] Avoid unnecessary (u)int32<->float64 changes in simplified lowering.

BUG=
R=bmeurer@chromium.org, titzer@chromium.org

Review URL: https://codereview.chromium.org/683873002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25045 ce2b1a6d-e550-0410-aec6-3dcde31c8c00