Make create_ll work with latest LLVM [NFC]

- Instead of running with -O0, we enable the highest optimization level, but
  then disable optimizations. This ensures that possibly important metadata
  is still emitted.

- Update the code for attribute removal to work with latest LLVM

- Do not cut an arbitrary number of lines from the LL file. It is undocumented
  why this was needed at the first place, and such a feature is likely to
  break with trivial IR changes that may come in the future.

llvm-svn: 307355