[llvm] [AMDGPU][MC] Disallow null as saddr in flat instructions (PR #101730)

[Jay Foad via llvm-commits]

jayfoad wrote:

> > Does this mean that `global_atomic_add v[1:2], v2, off` will work as before, but `global_atomic_add v[1:2], v2, null` won't be accepted anymore? This doesn't sound ideal if 'null' and 'off' are meant to be aliases (SP3 docs for GFX10 even say that 'off' is a legacy syntax and is translated to 'null').
> 
> The instruction `global_atomic_add v[1:2], v2, null` is not valid even without this patch. So already in this case `null` is not treated as an alias for `off`.

I agree that this patch does not break anything that currently works correctly. I also agree with Diana that it would be even better if `null` was treated the same as `off`, so that the assembler accepted `global_atomic_add v[1:2], v2, null`.

https://github.com/llvm/llvm-project/pull/101730