[PPC64] Add test case for r215685.

I had deferred adding this test case until I could get it down to a
reasonable size.  That's done now.

Thanks,
Bill


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215711 91177308-0d34-0410-b5e6-96231b3b80d8