enumFromTo for Integers goes via enumDeltaToInteger, which is less
efficient, as the "delta > = 0" check prevents more inlining which is
required for good fusion code. This rule avoids tihs check for the
common case of "delta = 1", makes up_fb visible and hence inlineable,
which greatly improves "length [n:Integer..m]"; even more so with
CallArity enabled. (#8766)