[llvm] Zext sext undef in sdag (PR #115451)

[via llvm-commits]

fengfeng09 wrote:

> What is the purpose behind this?
> 
> This approach is incorrect for how we handle extensions of undefs: https://alive2.llvm.org/ce/z/A4X7D5

In fact, there is no practical significance for now. But it is counterintuitive to evaluate extend undef to a zero. Especially, when we use undef value as a element in BUILD_VECTOR, it may produce some unexpected zeros.  

https://github.com/llvm/llvm-project/pull/115451