Allow duplicate calls to variable-like methods

It is common and recommended to create accessor methods and to extract code into accessor-like methods. The existing implementation of DuplicateMethodCall would force the result of these calls to be assigned to a local variable, if it was needed several times in a method. The resulting code would not be clearer.

This pull request remedies this.

With it, the following code:

```ruby
  attr_reader :bar

  def foo
    the_bar = bar
    case the_bar
    when :qux
      :baz
    else
      the_bar
    end
  end
```

can be written as:

```ruby
  attr_reader :bar

  def foo
    case bar
    when :qux
      :baz
    else
      bar
    end
  end
```