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

[Rahul Joshi via llvm-commits]

jurahul wrote:

@arsenm @jayfoad any feedback on the AMDGPU side? @topperc @s-barannikov @mshockwave would appreciate if you can PTAL as well.

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