[llvm] [AMDGPU] Enable serializing of allocated preload kernarg SGPRs info (PR #168374)

[Quentin Colombet via llvm-commits]

https://github.com/qcolombet commented:

I let @kerbowa give the final approval, but this looks good to me.
I've added a couple of comments.

https://github.com/llvm/llvm-project/pull/168374