add RPO to BB CFG cache

compiler/rustc_middle/src/mir/basic_blocks.rs

@@ -27,6 +27,7 @@ struct Cache {
     switch_sources: OnceCell<SwitchSources>,
     is_cyclic: OnceCell<bool>,
     postorder: OnceCell<Vec<BasicBlock>>,
+    reverse_postorder: OnceCell<Vec<BasicBlock>>,
     dominators: OnceCell<Dominators<BasicBlock>>,
 }
 
@@ -70,6 +71,17 @@ impl<'tcx> BasicBlocks<'tcx> {
         })
     }
 
+    /// Returns basic blocks in a reverse postorder.
+    #[inline]
+    pub fn reverse_postorder(&self) -> &[BasicBlock] {
+        self.cache.reverse_postorder.get_or_init(|| {
+            let mut rpo: Vec<_> =
+                Postorder::new(&self.basic_blocks, START_BLOCK).map(|(bb, _)| bb).collect();
+            rpo.reverse();
+            rpo
+        })
+    }
+
     /// `switch_sources()[&(target, switch)]` returns a list of switch
     /// values that lead to a `target` block from a `switch` block.
     #[inline]