Small change to Codegen prepare

Hi,

I'd like some feedback on patch https://reviews.llvm.org/D36504 that
makes a small change to codegenprepare. The patch duplicates returns
to try to enable more shrink-wrapping opportunities.
We loose some opportunities in the EarlyIfConverter with this change,
but end up with about 14% more functions shrink-wrapped in spec 2017
(on PowerPC). The patch breaks about 70 tests and most don't seem like
easy fixes. I'd like to get some feedback from the other targets, and
if the patch has a future I will go ahead and update the tests.

Thanks
Sean