[llvm] [AMDGPU] Move kernarg preload logic to AMDGPU Attributor (PR #123547)

[Shilei Tian via llvm-commits]

shiltian wrote:

>  I had a PR up to make AMDGPULowerKernelArguments a module pass, but I think the consensus was that it made sense to unify all the inreg/preload kernarg functionality here.

I remember that PR. I think that should be the way to go.

IIUC, @arsenm did mention in https://github.com/llvm/llvm-project/pull/101609 (which is indeed something about `inreg`) that we can probably move the logic _there_, but at that time I thought it runs part of the attributor iterations. I'm not sure if that is a good idea to pile everything in this file.

https://github.com/llvm/llvm-project/pull/123547