Represent `Result<usize, Box<T>>` as ScalarPair(i64, ptr) This allows types like `Result<usize, std::io::Error>` (and integers of differing sign, e.g. `Result<u64, i64>`) to be passed in a pair of registers instead of through memory, like `Result<u64, u64>` or `Result<Box<T>, Box<U>>` are today. Fixes #97540. r? `@ghost`