[llvm] [DAG] SimplifyDemandedBits - don't fold sext(x) -> aext(x) if we lose an 0/-1 allsignbits mask (PR #77296)

[Ulrich Weigand via llvm-commits]

uweigand wrote:

The SystemZ changes look good to me (and are a clear improvement).  Thanks!

https://github.com/llvm/llvm-project/pull/77296