----------------------------------------
	Be a bit less gung ho about let-floating
	----------------------------------------

Sometimes it's a bad idea to float cheap expressions
outwards, even if they escape a value lambda.

	  -- Even if it escapes a value lambda, we only
	  -- float if it's not cheap (unless it'll get all the
	  -- way to the top).  I've seen cases where we
	  -- float dozens of tiny free expressions, which cost
	  -- more to allocate than to evaluate.
	  -- NB: exprIsCheap is also true of bottom expressions, which
	  --     is good; we don't want to share them
	  --
	  -- It's only Really Bad to float a cheap expression out of a
	  -- strict context, because that builds a thunk that otherwise
	  -- would never be built.  So another alternative would be to
	  -- add
	  -- 	|| (strict_ctxt && not (exprIsBottom expr))
	  -- to the condition above. We should really try this out.

The relevant function is lvlMFE, which has been subject to a lot
of fiddling over the years.  Probably this isn't the last time.

This all actually showed up when I was compiling IA.lhs from Ian Lynagh.