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	Better arity stuff
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* CoreToStg now has a local function, predictArity, to
  predict the code-gen arity of a function. Better not to
  use CoreUtils.exprArity, because predictArity is a very
  local thing

* CoreUtils.exprArity is freed to do a better job.  Comments
  below.


exprArity is a cheap-and-cheerful version of exprEtaExpandArity.
It tells how many things the expression can be applied to before doing
any work.  It doesn't look inside cases, lets, etc.  The idea is that
exprEtaExpandArity will do the hard work, leaving something that's easy
for exprArity to grapple with.  In particular, Simplify uses exprArity to
compute the ArityInfo for the Id.

Originally I thought that it was enough just to look for top-level lambdas, but
it isn't.  I've seen this

	foo = PrelBase.timesInt

We want foo to get arity 2 even though the eta-expander will leave it
unchanged, in the expectation that it'll be inlined.  But occasionally it
isn't, because foo is blacklisted (used in a rule).

Similarly, see the ok_note check in exprEtaExpandArity.  So
	f = __inline_me (\x -> e)
won't be eta-expanded.

And in any case it seems more robust to have exprArity be a bit more intelligent.