We can save much code and explanation in Tag Inference and StgToCmm by making
`seq#` a known-key Magic Id in `GHC.Internal.IO` and inline this definition in
CorePrep. See the updated `Note [seq# magic]`.
I also implemented a new `Note [Flatten case-bind]` to get better code for
otherwise nested case scrutinees.

I renamed the contructors of `ArgInfo` to use an `AI` prefix in order to
resolve the clash between `type CpeApp = CoreExpr` and the data constructor of
`ArgInfo`, as well as fixed typos in `Note [CorePrep invariants]`.

Fixes #24252 and #24124.