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

[Rahul Joshi via llvm-commits]

jurahul wrote:

@topperc, @s-barannikov, @arsenm another friendly ping. It seems AMDGPU folks are fine (@jayfoad). Once its approved, I can hold off on committing for a few days in case downstream backends that are in sync with upstream need to prepare for it (I'll be doing the same for NVIDIA internal code). May be send a PSA on discourse as well if needed.

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