LLVM: opt arguments incompatible with upcoming 15 release
The arguments we pass to `opt` is incompatible with the upcoming 15 release, it'll fail with something like:
```
Cannot specify -O# and --passes=/--foo-pass, use -passes='default<O#>,other-pass'
`opt-15' failed in phase `LLVM Optimiser'. (Exit code: 1)
```
issue