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

[Jun Wang via llvm-commits]

jwanggit86 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.

Yes, I can do that.

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