Remove unnecessary mut.

`mut_results` immediately below is the `&mut self` version, this one
should be `&self`.

compiler/rustc_mir_dataflow/src/framework/cursor.rs

@@ -147,7 +147,7 @@ pub(crate) fn allow_unreachable(&mut self) {
     }
 
     /// Returns the underlying `Results`.
-    pub fn results(&mut self) -> &Results<'tcx, A, R::EntrySets> {
+    pub fn results(&self) -> &Results<'tcx, A, R::EntrySets> {
         self.results.borrow()
     }