[llvm] [DAG] canCreateUndefOrPoison/isGuaranteedNotToBeUndefOrPoison - SCALAR_TO_VECTOR upper elements are poison (PR #217185)

[Cyrus Ding via llvm-commits]

dingcyrus wrote:

> return POISON instead of UNDEF if we don't demand element0

Done — applied both suggestions: getUNDEF(VT) → getPOISON(VT) in the !DemandedElts[0] branches of both SimplifyDemandedBits and SimplifyDemandedVectorElts (and dropped the now-unneeded KnownUndef.setAllBits() in the latter). No further test changes required.

https://github.com/llvm/llvm-project/pull/217185