Address review comments on r290392:

- Don't break using '-mllvm -disable-llvm-optzns' (yet).
- Don't add support for '-mllvm -disable-llvm-passes'.

This is important for LLVM 4 as we haven't yet really told folks this is
coming. I'll add release notes about this.

I've also added some explicit testing of this so its more obvious what
is happening here.

llvm-svn: 291850