[DAG] Remove extract_vector_elt(freeze(x)), idx -> freeze(extract_vector_elt(x), idx) fold (#87480)

Reverse the fold with handling inside canCreateUndefOrPoison for cases where we know that the extract index is in bounds.

This exposed a number or regressions, and required some initial freeze handling of SCALAR_TO_VECTOR, which will require us to properly improve demandedelts support to handle its undef upper elements.

There is still one outstanding regression to be addressed in the future - how do we want to handle folds involving frozen loads?

Fixes #86968