[PowerPC] Fix VSX permutation isel

Not only did I invert the indices when I wrote the code, but I also did the
same thing when I wrote the regression test. Oops.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205046 91177308-0d34-0410-b5e6-96231b3b80d8