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     Buglet in the handling of unlifted bindings
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Unlifted bindings, like
	let I# v = ... in ...
can't be generalised.  In teh transition to GADTs I introduced a bug
that accidentally discarded some necessary dictionary bindings.

This commit fixes it by moving the test for unlifted bindings to a
much earlier point in tcBindWithSigs, which seems a lot cleaner to me.