Ignore errors re: unreachable blocks in dataflow cursor unit tests

compiler/rustc_mir_dataflow/src/framework/cursor.rs

@@ -64,6 +64,13 @@ where
         }
     }
 
+    /// Allows inspection of unreachable basic blocks even with `debug_assertions` enabled.
+    #[cfg(test)]
+    pub(crate) fn allow_unreachable(&mut self) {
+        #[cfg(debug_assertions)]
+        self.reachable_blocks.insert_all()
+    }
+
     /// Returns the underlying `Results`.
     pub fn results(&self) -> &Results<'tcx, A> {
         &self.results.borrow()