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In #18793, we saw a compelling example which requires us to look at non-recursive let-bindings during arity analysis and unleash their arity types at use sites. After the refactoring in the previous patch, the needed change is quite simple and very local to `arityType`'s defn for non-recurisve `Let`. Apart from that, we had to get rid of the second item of `Note [Dealing with bottoms]`, which was entirely a safety measure and hindered optimistic fixed-point iteration. Fixes #18793. The following metric increases are all caused by this commit and a result of the fact that we just do more work now: Metric Increase: T3294 T12545 T12707
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