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

[Hubert Tong via llvm-commits]

hubert-reinterpretcast wrote:

Unfortunately, I think the IR semantics do not say that we can do this.

For example: The result of sign-extending `undef` must leave the left (more significant) bits either all ones or all zeroes. Using `undef` as the result of the sign-extension does not retain that property. Another way to see it is that "valid" realizations of the wider `undef` may have values that are impossible as the result of extending from a narrower type.

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