Auto merge of #147619 - nnethercote:union-ne-check, r=<try>

Add a `!=` check to `ChunkedBitSet::union`.

Author: rust-bors[bot]

compiler/rustc_index/src/bit_set.rs

@@ -799,17 +799,26 @@ impl<T: Idx> BitRelations<ChunkedBitSet<T>> for ChunkedBitSet<T> {
                     Mixed(_other_chunk_count, other_chunk_words),
                 ) => {
                     // First check if the operation would change
-                    // `self_chunk.words`. If not, we can avoid allocating some
-                    // words, and this happens often enough that it's a
-                    // performance win. Also, we only need to operate on the
-                    // in-use words, hence the slicing.
+                    // `self_chunk.words`. (The `bitwise_changes` call alone
+                    // would answer this question, but first check if
+                    // `self_chunks_words` and `other_chunk_words` are unequal,
+                    // because that's a faster check than `bitwise_changes` and
+                    // very often succeeds.)
+                    //
+                    // If not, we can avoid doing the mutation, and this
+                    // avoidance happens often enough that it's a performance
+                    // win. Also, we only need to operate on the in-use words,
+                    // hence the slicing.
                     let op = |a, b| a | b;
                     let num_words = num_words(chunk_domain_size as usize);
-                    if bitwise_changes(
-                        &self_chunk_words[0..num_words],
-                        &other_chunk_words[0..num_words],
-                        op,
-                    ) {
+
+                    if self_chunk_words[0..num_words] != other_chunk_words[0..num_words]
+                        && bitwise_changes(
+                            &self_chunk_words[0..num_words],
+                            &other_chunk_words[0..num_words],
+                            op,
+                        )
+                    {
                         let self_chunk_words = Rc::make_mut(self_chunk_words);
                         let has_changed = bitwise(
                             &mut self_chunk_words[0..num_words],