[llvm] [llvm] Preserve existing calling conv when auto-upgrading nvvm.annota… (PR #125568)

[Alex MacLean via llvm-commits]

AlexMaclean wrote:

I agree that having `nvvm.annotations` on an `amdgpu_kernel` seems wrong/undefined and any front-end emitting these together should be updated to emit the target specific cc and remove usage of the `!"kernel"` annotation. 

[My understanding is that llvm maintains some backwards compatibility](https://llvm.org/docs/DeveloperPolicy.html#ir-backwards-compatibility). In this case since the original IR is malformed in my opinion and the current auto-upgrade rule seems like a reasonable way to choose between 2 conflicting pieces of information, I don't think this violates backwards compatibility. 

Is it possible to update whichever frontend is emitting this IR? If not, I'm open to switching to what you've proposed here, but I'd prefer the simper current solution unless there is a strong reason it will not work. 

https://github.com/llvm/llvm-project/pull/125568