This is completely unused upstream, and does not really have well defined semantics
on what this is supposed to do/how this fits into the ecosystem. Given that, as part of
splitting up the standard dialect it's best to just remove this behavior, instead of try
to awkwardly fit it somewhere upstream. Downstream users are encouraged to
define their own operations that clearly can define the semantics of this.
This also uncovered several lingering uses of ConstantOp that weren't
updated to use arith::ConstantOp, and worked during conversions because
the constant was removed/converted into something else before
See https://llvm.discourse.group/t/standard-dialect-the-final-chapter/ for more discussion.