Prefetch intrinsic

Add support for prefetch via llvm.prefetch.

This can/should be `span_err`, and it would be nice if you could provide a bit more information (e.g. which operation, and what the previous one was, that is, store the part that matched as well as the level, something like `|name: &str, param: &mut Option<(i32, &str)>, val|` called like `setparam("read-write", &mut rw, 0)`.