[llvm] [AMDGPU] Add function attribute to control TBUFFER combining (PR #156454)

[Harrison Hao via llvm-commits]

harrisonGPU wrote:

Hi, I’ve switched to using a module flag to control `RelaxedTBufferOOBMode`. When the mask has bit `0x2` set, `RelaxedTBufferOOBMode` is enabled. In the future, `RelaxedBufferOOBMode` will be enabled when the mask has bit `0x1` set.

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