smir: merge identical Constant and ConstOperand types
The first commit renames the const operand visitor functions on regular MIR to match the type name, that was forgotten in the original rename.
The second commit changes stable MIR, fixing https://github.com/rust-lang/project-stable-mir/issues/71. Previously there were two different smir types for the MIR type `ConstOperand`, one used in `Operand` and one in `VarDebugInfoContents`.
Maybe we should have done this with https://github.com/rust-lang/rust/pull/125967, so there's only a single breaking change... but I saw that PR too late.
Fixes https://github.com/rust-lang/project-stable-mir/issues/71
