[llvm] [AArch64][SME] Remove implicit-def's on smstart (PR #69012)

[Sander de Smalen via llvm-commits]

https://github.com/sdesmalen-arm commented:

Sorry for the delay in reviewing this patch, I've been catching up after getting back from some annual leave.

I think this patch makes sense. The issue seems to be caused by SelectionDAG, which adds the unnecessary implicit defs because of Glue (used for the smstart/smstop nodes), for which it makes some special assumptions.

Just an FYI that I've started looking into redesigning the smstart/smstop mechanism to simplify its design and also remove the need to use Glue in the DAG. That might make the code in this patch unnecessary in the future. But until we have something better, I'm happy moving forward with these changes.

https://github.com/llvm/llvm-project/pull/69012