[llvm] [DAG] NFC: Reword SCALAR_TO_VECTOR poison comment in SimplifyDemandedBits (PR #226003)

[Björn Pettersson via llvm-commits]

bjope wrote:

So this is a general rule for SimplifyDemandedVectorElts and SimplifyDemandedVectorEltsForTargetNode, that KnownUndef should exclude poison. Then I guess that should be documented in TargetLowering.h instead of being described specifically for SCALAR_TO_VECTOR.

But it also makes me confused since TargetLowering::SimplifyDemandedVectorElts has this:
```
  // Undef operand.
  if (Op.isUndef()) {
    KnownUndef.setAllBits();
    return false;
  }
```
which does include poison in KnownUndef propagation.

Does it really make sense that there is a special rule for SCALAR_TO_VECTOR? What am I missing here?

https://github.com/llvm/llvm-project/pull/226003