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

[Matt Arsenault via llvm-commits]

arsenm wrote:

>  In one case this leads to one extra argument being preloaded in a test. ISel correctly identifies this opportunity even when the IR pass previously missed it. 

Which case is this? Getting the exact number of arguments at all points should not be difficult 

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