这句话不是条件状语从句,
你可以这样理解这句话：
1： i don't care it.
2: he breaks his neck.
条件状语从句：
If you ask him, he will help you.
如果你请他帮忙,他会帮你的.
可以看到, 只有if 引导的 句子,才是条件状语从句.
而i don't care if he breaks his neck!
只是 I (主语) don't care （谓语）if he breaks his neck（宾语）.
其中,if只是宾语里面的条件状语从句.
整个句子就是这样.