Cleanup PPC64 unaligned i64 load/store

Remove an accidentally-added instruction definition and add a comment in the
test case. This is in response to a post-commit review by Bill Schmidt.

No functionality change intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177404 91177308-0d34-0410-b5e6-96231b3b80d8