Use BinOp::Cmp for iNN::signum

This way it can use the nice new LLVM intrinsic in LLVM20.

Author: scottmcm

library/core/src/intrinsics/mod.rs

@@ -3242,6 +3242,8 @@ pub const fn bitreverse<T: Copy>(_x: T) -> T {
 /// large and difficult to optimize.
 ///
 /// The stabilized version of this intrinsic is [`Ord::cmp`].
+#[rustc_intrinsic_const_stable_indirect]
+#[rustc_nounwind]
 #[rustc_intrinsic]
 #[rustc_intrinsic_must_be_overridden]
 pub const fn three_way_compare<T: Copy>(_lhs: T, _rhss: T) -> crate::cmp::Ordering {

library/core/src/num/int_macros.rs

@@ -3526,9 +3526,7 @@ macro_rules! int_impl {
             // exactly like we need and can be the place to deal with the complexity.
 
             // FIXME(const-hack): replace with cmp
-            if self < 0 { -1 }
-            else if self == 0 { 0 }
-            else { 1 }
+            crate::intrinsics::three_way_compare(self, 0) as Self
         }
 
         /// Returns `true` if `self` is positive and `false` if the number is zero or