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

[Thorsten Schütt via llvm-commits]

tschuett wrote:

There is prior art. I am not going to do git blame in public:
https://github.com/llvm/llvm-project/blob/e9be21786c972de93206599d4e06b79c2ca8f772/llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp#L414

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