[llvm] [AMDGPU] Make readonly noilas kernel arguments constant memory (PR #139629)

[Matt Arsenault via llvm-commits]

arsenm wrote:

I thought we did this in the distant past but it was incorrect. 

Code elsewhere, in another running kernel or the host could still be modifying the memory. The current langref doesn't have any explicit wording on access from other threads

https://github.com/llvm/llvm-project/pull/139629