[DAG] SimplifyDemandedBits - don't fold sext(x) -> aext(x) if we lose an 0/-1...

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

For targets that use 0/-1 boolean results, we want to keep this pattern through extensions/truncations as much as possible - so avoid simplifying to any_extend even if we don't demand the upper bits.

Noticed in triage for https://reviews.llvm.org/D152928