ConstOperand is duplicated and mir::Const is missing

[RalfJung]

In rustc, the main types related to consts are

• ty::Const: a const that appears in the type system e.g. an array length or a const generic. These must always be valtree-compatible.
• mir::Const: a const that appears in MIR. These can have arbitrary type.
• mir::ConstOperand: basically a Spanned<mir::Const> but also with some user_ty thing where I honestly don't know what it is for

smir has mir::ConstOperand as well. However it also has mir::Constant which is an exact duplicate of mir::ConstOperand up to field renaming! And furthermore, there is nothing that corresponds to rustc's mir::Const, which is odd.

[RalfJung]

With https://github.com/rust-lang/rust/pull/125967, mir::Const now exists as MirConst.