[clang] [llvm] [AMDGPU] Add per-kernel kernarg preload count (PR #211059)

[Yaxun Liu via cfe-commits]

yxsamliu wrote:

@arsenm Re: preload control

I changed the attribute to limit the SGPRs used by preloading instead of the number of arguments. This accounts for argument sizes and padding while matching the contiguous-prefix behavior supported by the backend today.

https://github.com/llvm/llvm-project/pull/211059