[llvm] Eliminating li of 0 into arg registers of unused arguments (PR #122741)

[zhijian lin via llvm-commits]

diggerlin wrote:

There are some formatters that failed in the check for code formatting: undef deprecator. I think this is related to my use of the SDValue SelectionDAG::getUNDEF(EVT VT) function in the patch.

```
// Lower poison to undef.
if (Ptr.getNode()->isPoison())
  Ptr = getUNDEF(Ptr.getValueType());
```

```
case ISD::POISON: {
  SDValue UndefNode = DAG.getUNDEF(Node->getValueType(0));
  ReplaceNode(Node, UndefNode.getNode());
  break;
}
```

If I can no longer use this function, the patch will become quite large. Can I still use getUNDEF in this patch while keeping the undef deprecator, and then introduce a follow-up patch to remove getUNDEF later? 

@hubert-reinterpretcast , @topperc 

https://github.com/llvm/llvm-project/pull/122741