You can use `HOMEBREW_PREFIX` or `Formula.factory('llvm')` here:

```ruby
system "\"#{HOMEBREW_PREFIX}/opt/llvm/bin/llvm-dis\" < foo.bc | grep foo_workgroup"

# or

system "\"#{Formula.factory('llvm').opt_prefix}/bin/llvm-dis\" < foo.bc | grep foo_workgroup"
```

Note that the path should be quoted too, in case the user's prefix contains spaces.