code: private Node tryFoldExpr(Node subtree) {
      Node result = trySimplifyUnusedResult(subtree.getFirstChild());
      if (result == null) {
        Node parent = subtree.getParent();
        // If the EXPR_RESULT no longer has any children, remove it as well.
        if (parent.isLabel()) {
          Node replacement = IR.block().srcref(subtree);
          parent.replaceChild(subtree, replacement);
          subtree = replacement;
        } else {
          subtree.detach();
          subtree = null;
        }
      }
      return subtree;
    }
docs: Try folding EXPR_RESULT nodes by removing useless Ops and expressions.
  @return the replacement node, if changed, or the original if not