Fix a regression in UselessAssignment with a single top level conditional

[yujinakayama]

This fixes a regression where UselessAssignment cop may not properly detect useless assignments when there's only a single conditional expression in the top level scope:

if flag
  foo = 1 # This is useless ...
else
  puts foo # ... since this will never run together with the assignment
end

This regression was introduced in 8eecfe0.

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Used the same coding conventions as the rest of the project.
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• All tests are passing.
• The new code doesn't generate RuboCop offenses.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Updated cop documentation with rake generate_cops_documentation (required only when you've added a new cop or changed the configuration/documentation of an existing cop).

[bbatsov]

The Jedi have returned! :-) Happy to see you back, @yujinakayama!

[yujinakayama]

I'm also happy 😀