[llvm] [DAG] Canonicalize zero_extend to sign_extend based on target preference (PR #70671)

[Philip Reames via llvm-commits]

preames wrote:

Abandoning as this is no longer applicable given the recent change to zext nneg.  The only piece of this which looks keeping, I've moved to it's own review and rewritten in terms of zext nneg.  https://github.com/llvm/llvm-project/pull/72281

https://github.com/llvm/llvm-project/pull/70671