Slightly simplify the iN::partial_cmp MIR

This saves some debug and scope metadata in every single function that calls it.

Normally wouldn't be worth it, but with the derives there's *so* many of these.

Author: scottmcm

library/core/src/cmp.rs

@@ -1548,7 +1548,14 @@ mod impls {
             impl PartialOrd for $t {
                 #[inline]
                 fn partial_cmp(&self, other: &$t) -> Option<Ordering> {
-                    Some(self.cmp(other))
+                    #[cfg(bootstrap)]
+                    {
+                        Some(self.cmp(other))
+                    }
+                    #[cfg(not(bootstrap))]
+                    {
+                        Some(crate::intrinsics::three_way_compare(*self, *other))
+                    }
                 }
                 #[inline(always)]
                 fn lt(&self, other: &$t) -> bool { (*self) < (*other) }
@@ -1566,12 +1573,12 @@ mod impls {
                 fn cmp(&self, other: &$t) -> Ordering {
                     #[cfg(bootstrap)]
                     {
-                    // 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 }
-                }
+                        // 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 }
+                    }
                     #[cfg(not(bootstrap))]
                     {
                         crate::intrinsics::three_way_compare(*self, *other)

tests/mir-opt/pre-codegen/derived_ord.{impl#0}-partial_cmp.PreCodegen.after.mir

@@ -16,24 +16,16 @@ fn <impl at $DIR/derived_ord.rs:6:10: 6:20>::partial_cmp(_1: &MultiField, _2: &M
     scope 2 (inlined std::cmp::impls::<impl PartialOrd for char>::partial_cmp) {
         debug self => _3;
         debug other => _4;
+        let mut _5: char;
+        let mut _6: char;
         let mut _7: std::cmp::Ordering;
-        scope 3 (inlined std::cmp::impls::<impl Ord for char>::cmp) {
-            debug self => _3;
-            debug other => _4;
-            let mut _5: char;
-            let mut _6: char;
-        }
     }
-    scope 4 (inlined std::cmp::impls::<impl PartialOrd for i16>::partial_cmp) {
+    scope 3 (inlined std::cmp::impls::<impl PartialOrd for i16>::partial_cmp) {
         debug self => _10;
         debug other => _11;
+        let mut _12: i16;
+        let mut _13: i16;
         let mut _14: std::cmp::Ordering;
-        scope 5 (inlined std::cmp::impls::<impl Ord for i16>::cmp) {
-            debug self => _10;
-            debug other => _11;
-            let mut _12: i16;
-            let mut _13: i16;
-        }
     }
 
     bb0: {