Add a generic operand visitor to `GenTree`.

Add a generic operand visitor to `GenTree`.

This visitor accepts any invokable argument (e.g. lambdas, functors,
etc.) with the following signature:

    `GenTree::VisitResult Visitor(GenTree* operand)`

This visitor is typically more efficient than the operand iterator
due to better opportunities for inlining and shorter dynamic path
lengths when deciding which operands need to be visited.