Add comment to avoid accidentally remove the changes.

Author: tesuji

src/libcore/cmp.rs

@@ -1012,6 +1012,8 @@ mod impls {
             impl Ord for $t {
                 #[inline]
                 fn cmp(&self, other: &$t) -> Ordering {
+                    // The order here is important to generate more optimal assembly.
+                    // See <https://github.com/rust-lang/rust/issues/63758> for more info.
                     if *self < *other { Less }
                     else if *self > *other { Greater }
                     else { Equal }