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

[Craig Topper via llvm-commits]

topperc wrote:

This optimization is not correct for 'undef'. I think it would be correct for 'poison', but SelectionDAG doesn't have 'poison' today.

See also https://blog.regehr.org/archives/1837

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