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

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

> Do you know why this isn't a problem for other DAG combiners? Was it the only one creating a ISD::CONSTANT directly?

It depends on if there's already an existing zero ISD::Constant already in the DAG. If there is it'll lower correctly (as that constant will be in the worklist), if there's not the lowering will fail. You can see this here https://godbolt.org/z/v3TxMn83z (the only difference is `ret i32 0` vs `ret i32 1`, which in theory should not change the lowering of an unrelated node...)

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