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

[Diana Picus via llvm-commits]

rovka wrote:

> The instruction `global_atomic_add v[1:2], v2, null` is not valid even without this patch. 

Ok, in that case I'm happy with this patch after you address Jay's concern. It would be nice to follow up with another patch making `null` work wherever `off` works, if you have the time.



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