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

[Diana Picus via llvm-commits]

rovka 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'). 

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