Make let-matching work in Rules again

A RULE is supposed to match even if there is an intervening let:
	RULE f (x:xs) = ....
	target	   f (let x = thing in x:xs)

It's surprisingly tricky to get this right; in effect we are doing
let-floating on the fly. I managed to get it wrong before, or at least
be over-conservative.  And in "fixing" that I got it wrong again in a
different way, which made it far too conservative. In particular, it
failed to match	f (let x = y+y in let z=x+y in z:xs)
because the binder x was cloned and looked "locally-bound". See the
ever growing comments with the Let rule for details.

That patch reverts to the previous story, which is still a bit too 
conservative, but not so egregiously so.  Fixes Romans's problem.

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