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

[Krzysztof Drewniak via llvm-commits]

krzysz00 wrote:

If it helps, this is part of letting buffer_load become s_buffer_load, where IIRC there were concerns about cache conference with loads that weren't from invariant memory 

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