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

[Thorsten Schütt via llvm-commits]

tschuett wrote:

I stated several times before if we remove the legality check from the replaceInstWithConstant function, then we have to verify that on all paths to the function the correct legality checks were made. Today the combiner is spread over several files. This is error-prone.

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