[llvm] [GlobalISel] Combine [a, s, z]ext of undef into 0 or undef (PR #117439)

[Amara Emerson via llvm-commits]

aemerson wrote:

> I need some help to understand that request. When did we stop checking for legality before building?

When did we start checking for legality of undef is the better question. Do you have a technical rebuttal to the specific case of undef? If not there's no point continuing this discussion.

https://github.com/llvm/llvm-project/pull/117439