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

[Shilei Tian via llvm-commits]

https://github.com/shiltian commented:

> Kernels produced by ROCm classic flang may have both nvvm "kernel" annotation and amdgpu_kernel calling convention.

I think this should be fixed in classic flang instead of here. This is treating symptoms instead of root cause.

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