Refactor checks to improve codegen.

src/mem/freelist.h

@@ -139,10 +139,9 @@ namespace snmalloc
 #  ifndef NDEBUG
       if (next != nullptr)
       {
-        if (unlikely(different_slab(prev, next)))
-        {
-          error("Heap corruption - free list corrupted!");
-        }
+        check_client(
+          !different_slab(prev, next),
+          "Heap corruption - free list corrupted!");
       }
 #  endif
       prev = address_cast(curr);
@@ -162,10 +161,8 @@ namespace snmalloc
     void move_next()
     {
 #ifdef CHECK_CLIENT
-      if (unlikely(different_slab(prev, curr)))
-      {
-        error("Heap corruption - free list corrupted!");
-      }
+      check_client(
+        !different_slab(prev, curr), "Heap corruption - free list corrupted!");
 #endif
       update_cursor(curr->read_next(get_prev()));
     }
@@ -221,6 +218,14 @@ namespace snmalloc
       return front.get_curr() == nullptr;
     }
 
+    /**
+     * Returns current head without affecting the iterator.
+     */
+    void* peek()
+    {
+      return front.get_curr();
+    }
+
     /**
      * Moves the iterator on, and returns the current value.
      */
@@ -258,7 +263,7 @@ namespace snmalloc
      */
     void* peek_head()
     {
-      return front.get_curr();
+      return peek();
     }
 
     /**