remove unused postorder CFG cache

Author: lqd

compiler/rustc_middle/src/mir/basic_blocks.rs

@@ -26,7 +26,6 @@ struct Cache {
     predecessors: OnceCell<Predecessors>,
     switch_sources: OnceCell<SwitchSources>,
     is_cyclic: OnceCell<bool>,
-    postorder: OnceCell<Vec<BasicBlock>>,
     reverse_postorder: OnceCell<Vec<BasicBlock>>,
     dominators: OnceCell<Dominators<BasicBlock>>,
 }
@@ -63,14 +62,6 @@ impl<'tcx> BasicBlocks<'tcx> {
         })
     }
 
-    /// Returns basic blocks in a postorder.
-    #[inline]
-    pub fn postorder(&self) -> &[BasicBlock] {
-        self.cache.postorder.get_or_init(|| {
-            Postorder::new(&self.basic_blocks, START_BLOCK).map(|(bb, _)| bb).collect()
-        })
-    }
-
     /// Returns basic blocks in a reverse postorder.
     #[inline]
     pub fn reverse_postorder(&self) -> &[BasicBlock] {