[llvm] [AArch64] Avoid creating a new generic constant in SelectSMETileSlice (PR #203344)

[Gaëtan Bossu via llvm-commits]

https://github.com/gbossu approved this pull request.

Thanks for the explanations, it makes sense now. It's really interesting that basically DAG-level CSE saved us in most cases 😆 

> Note: This not a DAG combiner, it's a complex pattern that executes during instruction selection (after all legalization/DAG combines).

I think this is worth noting in `getZeroRegister()`: the helper can get called after legalisation has happened, and we are expected to emit an `SDNode` that is "already selected".

https://github.com/llvm/llvm-project/pull/203344