Split branches in heapsort child selection

This allows even better code-gen, cmp + adc. While also more clearly
communicating the intent.

library/core/src/slice/sort.rs

@@ -198,7 +198,12 @@ where
             }
 
             // Choose the greater child.
-            child += (child + 1 < v.len() && is_less(&v[child], &v[child + 1])) as usize;
+            if child + 1 < v.len() {
+                // We need a branch to be sure not to out-of-bounds index,
+                // but it's highly predictable.  The comparison, however,
+                // is better done branchless, especially for primitives.
+                child += is_less(&v[child], &v[child + 1]) as usize;
+            }
 
             // Stop if the invariant holds at `node`.
             if !is_less(&v[node], &v[child]) {