[TableGen] Restore OpName::OPERAND_LAST emission in InstrInfoEmitter (#125265)
- Looks like this sentinel value is used in some downstream backends, so
restore emitting it.
- It now also has the correct value (earlier code may have emitted an
incorrect value for OPERAND_LAST and hence it was removed in
https://github.com/llvm/llvm-project/pull/124960)