formerly rejected by the FE, so asserted in the BE; now the FE only
warns, so we treat it as a legitimate fatal error in PPC BE.
This means the test for the feature won't pass, so it's xfail'd.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@109892 91177308-0d34-0410-b5e6-96231b3b80d8