ApplicativeDo should use *> and <*

From @ekmett:

Would it be possible to tweak the generation to use (<*) or (*>) where appropriate when the result isn't being used?

For many Applicatives this can be a massive asymptotic win in terms of sharing and/or computational cost.

When desugaring using (<*) you'd just omit any handling of the unused result instead.

(\x y -> ...) <$> foo <* bar <*> baz

corresponds to

do x <- foo
  bar
  y <- baz
  return ...

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Trac field	Value
Version	7.11
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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