Teach DmdAnal that coercions are value arguments!
The demand analyser was treating coercion args like type args, which meant that the arguments in a strictness signature got out of step with the arguments of a call. Result chaos and disaster. Trac #10288 showed it up. It's hard to get this bug to show up in practice because - functions abstracted over coercions are usually abstracted over *boxed* coercions - we don't currently unbox a boxed-coercion arg because it's GADT (I see how to fix this too) But after floating, optimisation, and so on, Trac #10288 did get a function abstracted over an unboxed coercion, and then the -flate-dmd-anal pass went wrong. I don't think I can come up with a test case, but I don't think it matters too much. Still to come - Fix a second bug, namely that coercion variables are wrongly marked as absent because DmdAnal doesn't check the the free variables of casts. I think this never bites in practice (see the follow-up commit) - Make GADT products work with strictness analysis
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