Code pushing support for br_on_exn (#2660)

Like `br_if`, `br_on_exn` is a conditional branch and across which code
can be pushed past when conditions are satisfied.

Also adds a few lines of comments and NFC changes in a couple places.
Changes in Vacuum are NFC because they were being handled in `default:`
in the same way anyway, but I added them to be more explicit and
consistent with existing code.

4 files changed, 7 insertions, 2 deletions

diff --git a/src/dataflow/graph.h b/src/dataflow/graph.h
index 41b145ee9..02679995b 100644
--- a/src/dataflow/graph.h
+++ b/src/dataflow/graph.h
@@ -198,6 +198,8 @@ struct Graph : public UnifiedExpressionVisitor<Graph, Node*> {
   // Visiting.
 
   Node* visitExpression(Expression* curr) {
+    // TODO Exception handling instruction support
+
     // Control flow and get/set etc. are special. Aside from them, we just need
     // to do something very generic.
     if (auto* block = curr->dynCast<Block>()) {
diff --git a/src/passes/CodePushing.cpp b/src/passes/CodePushing.cpp
index 6907f4fdc..7fa50f39b 100644
--- a/src/passes/CodePushing.cpp
+++ b/src/passes/CodePushing.cpp
@@ -141,7 +141,7 @@ private:
     if (auto* drop = curr->dynCast<Drop>()) {
       curr = drop->value;
     }
-    if (curr->is<If>()) {
+    if (curr->is<If>() || curr->is<BrOnExn>()) {
       return true;
     }
     if (auto* br = curr->dynCast<Break>()) {
diff --git a/src/passes/Vacuum.cpp b/src/passes/Vacuum.cpp
index 54707fddc..9b67a3a16 100644
--- a/src/passes/Vacuum.cpp
+++ b/src/passes/Vacuum.cpp
@@ -80,9 +80,12 @@ struct Vacuum : public WalkerPass<ExpressionStackWalker<Vacuum>> {
           return curr; // not always needed, but handled in visitLoop()
         case Expression::Id::DropId:
           return curr; // not always needed, but handled in visitDrop()
+        case Expression::Id::TryId:
+          return curr; // not always needed, but handled in visitTry()
 
         case Expression::Id::BreakId:
         case Expression::Id::SwitchId:
+        case Expression::Id::BrOnExnId:
         case Expression::Id::CallId:
         case Expression::Id::CallIndirectId:
         case Expression::Id::LocalSetId:
diff --git a/src/wasm-traversal.h b/src/wasm-traversal.h
index eae4634b5..ee6d22aa5 100644
--- a/src/wasm-traversal.h
+++ b/src/wasm-traversal.h
@@ -1159,7 +1159,7 @@ struct ControlFlowWalker : public PostWalker<SubType, VisitorType> {
           return curr;
         }
       } else {
-        // an if, ignorable
+        // an if or try, ignorable
         assert(curr->template is<If>() || curr->template is<Try>());
       }
       if (i == 0) {