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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> I think it is not same as `relaxed-buffer-oob-mod`, this attribute is aims to the Vulkan semantics for typed-buffers and unbound vertex attributes, where if any component is out of bounds, the entire result must return zero.So the attribute is not enabling relaxed OOB handling, but instead enforcing the stricter API-level semantics which prohibit TBUFFER combine.

I mean what you proposed sounds like the opposite of relaxed-buffer-oob-mod. i.e. you don't need something new, the pass is just bugged and needs to check if relaxed-buffer-oob-mod is enabled to perform the merge 

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