slot2ssa: remove `TypedSlot` (#50943)
Dependent on https://github.com/JuliaLang/julia/pull/50924 .
This removes the last remaining usage of `TypedSlot`.
As a bonus, this evicts all of the "tmerge" type-iteration that was
being done in slot2ssa.
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Co-authored-by: Shuhei Kadowaki <aviatesk@gmail.com>
