[llvm] [CodeGen][NPM] Support generic regalloc-npm option (PR #135149)

[Akshat Oke via llvm-commits]

optimisan wrote:

> > Right now other passes can be run here (will need to add a way to mark a pass as a regalloc pass) If `-passes` is used, this argument will be ignored.
> 
> Ignore is bad. If we're going to have this embedded flag, it needs to be a hard error that you can't mix the flags.

Updated so that `--passes` and `--regalloc-npm` cannot be both specified and only regalloc passes can be added in the `--regalloc-npm` option.

https://github.com/llvm/llvm-project/pull/135149