Temporarily XFAIL fallout from r225083 while investigating.

Between this behavior and that fixed by r225083/r225000, I'll take the
latter over the former for now, but I'm immediately working on
understanding/addressing this behavior too.

(the fact that the code change in r225083 caused this change in behavior
is a bit troubling anyway - given that it looks & claims to be just a
preformance thing)

llvm-svn: 225086