[llvm] [GlobalISel] Combine G_MERGE_VALUES of x and undef (PR #113616)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

> Why do do ZEXT instead of ANYEXT? Some special poison semantics?

My model of G_MERGE_VALUES is that the sources are memcopied into the destination. Sext would not fit into that model.

https://github.com/llvm/llvm-project/pull/113616