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

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macurtis-amd wrote:

> > 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.

I considered that, but thought that this fix would be better since there may be existing bitcode files with the condition.

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