Adicionando as constraints, isso pode ser colocado dentro de uma função, será chamado dentro da classe LoginViewController.

LoginViewController.swift

loginLabel.centerYAnchor.constraint(equalTo: self.view.safeAreaLayoutGuide.centerYAnchor, constant: -100).isActive = true
loginLabel.leadingAnchor.constraint(equalTo: self.view.leadingAnchor, constant: 10).isActive = true
loginLabel.trailingAnchor.constraint(equalTo: self.loginTextField.leadingAnchor, constant: -10).isActive = true
loginLabel.heightAnchor.constraint(equalToConstant: 40).isActive = true

loginTextField.centerYAnchor.constraint(equalTo: self.view.safeAreaLayoutGuide.centerYAnchor, constant: -100).isActive = true
loginTextField.trailingAnchor.constraint(equalTo: self.view.trailingAnchor, constant: -10).isActive = true
loginTextField.leadingAnchor.constraint(equalTo: self.loginLabel.trailingAnchor, constant: 10).isActive = true
loginTextField.widthAnchor.constraint(equalTo:self.loginLabel.widthAnchor).isActive = true
loginTextField.heightAnchor.constraint(equalToConstant: 40).isActive = true

passwordLabel.topAnchor.constraint(equalTo: self.loginLabel.bottomAnchor, constant: 30).isActive = true
passwordLabel.leadingAnchor.constraint(equalTo: self.view.leadingAnchor, constant: 10).isActive = true
passwordLabel.trailingAnchor.constraint(equalTo: self.loginTextField.leadingAnchor, constant: -10).isActive = true
passwordLabel.heightAnchor.constraint(equalToConstant: 40).isActive = true

passwordTextField.topAnchor.constraint(equalTo: self.loginTextField.bottomAnchor, constant: 30).isActive = true
passwordTextField.trailingAnchor.constraint(equalTo: self.view.trailingAnchor, constant: -10).isActive = true
passwordTextField.leadingAnchor.constraint(equalTo: self.loginLabel.trailingAnchor, constant: 10).isActive = true
passwordTextField.widthAnchor.constraint(equalTo:self.loginLabel.widthAnchor).isActive = true
passwordTextField.heightAnchor.constraint(equalToConstant: 40).isActive = true

loginButton.bottomAnchor.constraint(equalTo: self.view.bottomAnchor, constant: -100).isActive = true
loginButton.trailingAnchor.constraint(equalTo: self.view.trailingAnchor, constant: -10).isActive = true
loginButton.leadingAnchor.constraint(equalTo: self.view.leadingAnchor, constant: 10).isActive = true