Micro-optimize Ord::cmp for primitives

Author: saethlin

library/core/src/cmp.rs

@@ -1476,11 +1476,11 @@ mod impls {
             impl const 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 { Equal }
-                    else { Greater }
+                    let mut res = 0i8;
+                    res -= (*self < *other) as i8;
+                    res += (*self > *other) as i8;
+                    // SAFETY: The discriminants of Ord were chosen to permit this
+                    unsafe { crate::mem::transmute(res) }
                 }
             }
         )*)