Merge: Warn useless do block

Warn useless do block, like

~~~nit
loop do
  print 1
end
~~~

Close #1437

Pull-Request: #1438
Reviewed-by: Romain Chanoir <chanoir.romain@courrier.uqam.ca>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>