Fix CFGWalker issue in single-threaded mode (#6573)

In that mode a walk on an entire module will reuse the same CFGWalker
instance, so we must manually clear some fields, and we forgot some
before.

src/cfg/cfg-traversal.h

@@ -548,6 +548,8 @@ struct CFGWalker : public PostWalker<SubType, VisitorType> {
   void doWalkFunction(Function* func) {
     basicBlocks.clear();
     debugIds.clear();
+    exit = nullptr;
+    hasSyntheticExit = false;
 
     startBasicBlock();
     entry = currBasicBlock;