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

[Stanislav Mekhanoshin via llvm-commits]

rampitec 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

Yes, const only guaranties it is not modified in this function. Arguably one should use atomics for cross-thread communication though.

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