[llvm] [GlobalISel] Combine G_ZEXT of undef -> 0 (PR #113764)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

> There is a bug in the repo. We do ` Builder.buildConstant(MI.getOperand(0), C);` without checking for legality. This PR fixes the bug in the correct way, which is also the long-term correct solution. It just puts a legality check on top. For readers, it is easy to understand, there is a legality check before the build.
> 
> Exploding PRs based on misunderstanding is a poor sport.

You are ignoring  my comment. No surprise.

https://github.com/llvm/llvm-project/pull/113764