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

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

That Op.isUndef() needs changing:
```
if (Op.isUndef()) {
   if (Op.getOpcode() == ISD::UNDEF)
      KnownUndef.setAllBits();
    return false;
  }
```

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