Cranelift: Add a helper for getting a block's successors (#9067)

Co-authored-by: Trevor Elliott <Telliott@fastly.com>

cranelift/codegen/src/ir/function.rs

@@ -313,6 +313,16 @@ impl FunctionStencil {
         Ok(())
     }
 
+    /// Returns an iterator over the blocks succeeding the given block.
+    pub fn block_successors(&self, block: Block) -> impl DoubleEndedIterator<Item = Block> + '_ {
+        self.layout.last_inst(block).into_iter().flat_map(|inst| {
+            self.dfg.insts[inst]
+                .branch_destination(&self.dfg.jump_tables)
+                .iter()
+                .map(|block| block.block(&self.dfg.value_lists))
+        })
+    }
+
     /// Returns true if the function is function that doesn't call any other functions. This is not
     /// to be confused with a "leaf function" in Windows terminology.
     pub fn is_leaf(&self) -> bool {

cranelift/codegen/src/traversals.rs

@@ -96,11 +96,9 @@ impl Iterator for DfsIter<'_> {
 
         if event == Event::Enter && self.dfs.seen.insert(block) {
             self.dfs.stack.push((Event::Exit, block));
-            if let Some(inst) = self.func.layout.last_inst(block) {
             self.dfs.stack.extend(
-                    self.func.dfg.insts[inst]
+                self.func
-                        .branch_destination(&self.func.dfg.jump_tables)
+                    .block_successors(block)
-                        .iter()
                     // Heuristic: chase the children in reverse. This puts
                     // the first successor block first in the postorder, all
                     // other things being equal, which tends to prioritize
@@ -111,7 +109,6 @@ impl Iterator for DfsIter<'_> {
                     // purely for other consumers of the postorder we cache
                     // here.
                     .rev()
-                        .map(|block| block.block(&self.func.dfg.value_lists))
                     // This is purely an optimization to avoid additional
                     // iterations of the loop, and is not required; it's
                     // merely inlining the check from the outer conditional
@@ -121,7 +118,6 @@ impl Iterator for DfsIter<'_> {
                     .map(|block| (Event::Enter, block)),
             );
         }
-        }
 
         Some((event, block))
     }