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

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

Actually, thinking about it a bit more, special-casing poison inputs simply doesn't work.  We lower poison-producing operations to non-poison-producing operations, so you can't actually prove whether an operation originally produced poison.

Maybe we just want to transform `freeze(assertzext(x)) -> freeze(x)`?

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