Revert change to clone_from.

Author: cjgillot

compiler/rustc_index/src/bit_set.rs

@@ -1078,12 +1078,8 @@ impl<T> Clone for BitSet<T> {
     }
 
     fn clone_from(&mut self, from: &Self) {
-        if self.domain_size == from.domain_size {
-            bitwise(&mut self.words, &from.words, |_, b| b);
-        } else {
-            self.domain_size = from.domain_size;
-            self.words.clone_from(&from.words);
-        }
+        self.domain_size = from.domain_size;
+        self.words.clone_from(&from.words);
     }
 }