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

[Hubert Tong via llvm-commits]

hubert-reinterpretcast wrote:

> According to InstCombine, (sext/zext poison) can constant fold to poison in the larger type.

It sounds like you have more context than I do then. I believe that the description of `sext`, `zext`, etc. in the IR will benefit from this clarification. Are you open to posting a PR?

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