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

[Matt Arsenault via llvm-commits]

================
@@ -1214,7 +1214,8 @@ class SIInstrInfo final : public AMDGPUGenInstrInfo {
   /// threads in the wave.
   /// \returns The SGPR register that \p SrcReg was copied to.
----------------
arsenm wrote:

Comment the parameter 

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