[llvm] [TableGen] Emit OpName as an enum class instead of a namespace (PR #125313)

[Rahul Joshi via llvm-commits]

jurahul wrote:

> > * Add OpName::NUM_OPERAND_NAMES as an alias for OPERAND_LAST (to be deprecated).
> 
> I don't think it needs to be deprecated, this is a trivial change.

Done.

https://github.com/llvm/llvm-project/pull/125313