Replace with_imm_node by with_base and with_mut_node by with_mut_base

This is a refactoring patch that replaces `with_imm_node` by `with_base` and `with_mut_node` by `with_mut_base`. The intention is that the new names better reflect how we actually use these methods. In the case of `with_base` the name is also shorter, which is nice.