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.

Commit migrated from https://github.com/dotnet/coreclr/commit/9f9a7efe3950c8cb22ffdfc7074e26c350bb4b6a