add comment explaining optimization Filter::next_chunk

Author: the8472

library/core/src/iter/adapters/filter.rs

@@ -41,8 +41,9 @@ where
 
         let result = self.iter.try_for_each(|element| {
             let idx = initialized;
+            // branchless index update combined with unconditionally copying the value even when
+            // it is filtered reduces branching and dependencies in the loop.
             initialized = idx + (self.predicate)(&element) as usize;
-
             // SAFETY: Loop conditions ensure the index is in bounds.
             unsafe { array.get_unchecked_mut(idx) }.write(element);