[PowerPC] Turn on CR-Logical reducer pass

Quite a while ago, we implemented a pass that will reduce the number of
CR-logical operations we emit. It does so by converting a CR-logical operation
into a branch. We have kept this off by default because it seemed to cause a
significant regression with one benchmark.
However, that regression turned out to be due to a completely unrelated
reason - AADB introducing a self-copy that is a priority-setting nop and it was
just exacerbated by this pass.

Now that we understand the reason for the only degradation, we can turn this
pass on by default. We have long since fixed the cause for the degradation.

Differential revision: https://reviews.llvm.org/D52431

llvm-svn: 375152