[llvm] using UNDEF SDNode instead of Constant SDNode when extend the UNDEF SDNode (PR #122741)

[Hubert Tong via llvm-commits]

hubert-reinterpretcast wrote:

> but according to the https://releases.llvm.org/11.0.0/docs/LangRef.html#poison-values

The "up-to-date" version of the document is here: https://llvm.org/docs/LangRef.html#poison-values.
It indicates that `poison` can be used like a constant.

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