[llvm] [DAG] Add freeze(assertext(x)) -> assertext(x) folds (PR #94491)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

What's the advantage of "if the input is poison, the result is poison; otherwise, if the input isn't zero-extended the behavior is undefined", vs. the much simpler "if the input isn't zero-extended, the result is poison".

https://github.com/llvm/llvm-project/pull/94491