[llvm] [AMDGPU] Add function attribute to disable TBUFFER combine (PR #156454)

[Piotr Sobczak via llvm-commits]

piotrAMD wrote:

It appears the only remaining point of contention is whether to use a module flag or a function attribute. I agree with Matt that making it module level could introduce new problems. I prefer using a function attribute, which is also more flexible if we decide to set it on a per-function basis in the future.

https://github.com/llvm/llvm-project/pull/156454