The type extraction helper function for block argument and op result
list objects was ignoring the slice entirely. So was the slice addition.
Both are caused by a misleading naming convention to implement slices
via CRTP. Make the convention more explicit and hide the helper
functions so users have harder time calling them directly.
Closes#56540.
Reviewed By: stellaraccident
Differential Revision: https://reviews.llvm.org/D130271
ee168fb90e