Fix an incorrect autocorrect for Lint/RegexpAsCondition

[koic]

This PR fixes the following incorrect autocorrect for Lint/RegexpAsCondition when using regexp literal with bang:

if !/foo/ =~ $_ =~ $_
end

% be rubocop --only Lint/RegexpAsCondition -a
(snip)

Offenses:

example.rb:1:5: W: [Corrected] Lint/RegexpAsCondition: Do not use regexp literal as a condition. The regexp literal matches $_ implicitly.
if !/foo/
    ^^^^^

1 file inspected, 1 offense detected, 1 offense corrected

% git diff .
diff --git a/regexp_as_condition/example.rb b/regexp_as_condition/example.rb
index 3cb37b3..6f925a3 100644
--- a/regexp_as_condition/example.rb
+++ b/regexp_as_condition/example.rb
@@ -1,2 +1,2 @@
-if !/foo/
+if !/foo/ =~ $_ =~ $_
 end

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[bbatsov]

Thanks!