[Fix #1230] Fix a false positive for Rails/SaveBang if persisted? is checked on parenthesised expression.

[Earlopain]

There are a few more cases I can think of that should potentially be handled, like

while (post = User.create(params).persisted?
  params = new_params
end

but I'd wait until someone actually complains. It doesn't look like very practical code to me.

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