Stop using HybridBitSet in dataflow diffs.

As part of the larger goal of reducing `HybridBitSet` use in general.
This code is for debugging only and isn't performance sensitive, so
`ChunkedBitSet` should be fine.

Author: nnethercote

compiler/rustc_mir_dataflow/src/framework/fmt.rs

@@ -4,7 +4,7 @@
 use std::fmt;
 
 use rustc_index::Idx;
-use rustc_index::bit_set::{BitSet, ChunkedBitSet, HybridBitSet};
+use rustc_index::bit_set::{BitSet, ChunkedBitSet};
 
 use super::lattice::MaybeReachable;
 
@@ -85,8 +85,8 @@ where
         let size = self.domain_size();
         assert_eq!(size, old.domain_size());
 
-        let mut set_in_self = HybridBitSet::new_empty(size);
-        let mut cleared_in_self = HybridBitSet::new_empty(size);
+        let mut set_in_self = ChunkedBitSet::new_empty(size);
+        let mut cleared_in_self = ChunkedBitSet::new_empty(size);
 
         for i in (0..size).map(T::new) {
             match (self.contains(i), old.contains(i)) {
@@ -112,8 +112,8 @@ where
         let size = self.domain_size();
         assert_eq!(size, old.domain_size());
 
-        let mut set_in_self = HybridBitSet::new_empty(size);
-        let mut cleared_in_self = HybridBitSet::new_empty(size);
+        let mut set_in_self = ChunkedBitSet::new_empty(size);
+        let mut cleared_in_self = ChunkedBitSet::new_empty(size);
 
         for i in (0..size).map(T::new) {
             match (self.contains(i), old.contains(i)) {
@@ -159,8 +159,8 @@ where
 }
 
 fn fmt_diff<T, C>(
-    inserted: &HybridBitSet<T>,
-    removed: &HybridBitSet<T>,
+    inserted: &ChunkedBitSet<T>,
+    removed: &ChunkedBitSet<T>,
     ctxt: &C,
     f: &mut fmt::Formatter<'_>,
 ) -> fmt::Result