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

[Simon Pilgrim via llvm-commits]

RKSimon 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

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