[PowerPC] Add another test for load/store with update

We now produce the desired code as noted in the README.txt file. Remove the
README entry and add a regression test.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225205 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed