Use sparse representation of switch sources

to avoid quadratic space overhead

compiler/rustc_mir_dataflow/src/framework/direction.rs

@@ -322,7 +322,7 @@ where
     fn apply(&mut self, mut apply_edge_effect: impl FnMut(&mut D, SwitchIntTarget)) {
         assert!(!self.effects_applied);
 
-        let values = &self.body.switch_sources()[self.bb][self.pred];
+        let values = &self.body.switch_sources()[&(self.bb, self.pred)];
         let targets = values.iter().map(|&value| SwitchIntTarget { value, target: self.bb });
 
         let mut tmp = None;