Cleanup PPCEarlyReturn

Some general cleanup and only scan the end of a BB for branches (once we're
done with the terminators and debug values, then there should not be any other
branches). These address post-commit review suggestions by Bill Schmidt.

No functionality change intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179112 91177308-0d34-0410-b5e6-96231b3b80d8