[Bug rtl-optimization/57231] Hoist zero-extend operations when possible

[josh.m.conner at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57231

--- Comment #3 from Joshua Conner <josh.m.conner at gmail dot com> ---
Exactly - there's no need to truncate every iteration, we should be able to
safely do it when the loop is complete.