[llvm] [AMDGPU] Enable kernarg preloading by default on gfx940 (PR #110691)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm requested changes to this pull request.

Should add some aggregate tests, and just skip them if you just want to handle the simple register cases 

https://github.com/llvm/llvm-project/pull/110691