[llvm] [AMDGPU] Extend zero initialization of return values for TFE (PR #85759)

[David Stuttard via llvm-commits]

dstutt wrote:

> > There's no global isel support implemented yet - but I thought that could maybe be a separate change? (I'm not sure about the best approach currently).
> 
> I think it's best to do them together, otherwise they bifurcate

Yes, I thought you might say that :)
I'll take another look.
Any suggestions? Image support is done in AMDGPUInstructionSelector.cpp:selectImageIntrinsic - so not appropriate for the buffer instructions - but it seems like overkill to implement a selectBufferLoadTfe when selectImpl does a good job already (but maybe that's the best approach??)

https://github.com/llvm/llvm-project/pull/85759