[llvm] [Offload][AMDGPU] Correctly handle variable implicit argument sizes (PR #142199)

[Jon Chesterfield via llvm-commits]

JonChesterfield wrote:

Hold on, what "optimisation" is changing this struct? It's exposed in the ABI, in the literal sense, so LLVM really should not be hacking around with it.

https://github.com/llvm/llvm-project/pull/142199