Teach DmdAnal about free coercion variables
Coercion variables are used in casts and coercions, so the demand analyser should jolly well not regard them as absent! In fact this bug never makes a difference because even absent unboxed-coercion arguments are passed anyway; see WwLib.mk_abesnt_let, which returns Nothing for coercion Ids. But it was simply wrong before and that is never cool.
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