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

[Thorsten Schütt via llvm-commits]

tschuett wrote:

With G_ANYEXT, there are almost no changes. Perhaps G_MERGE_VALUES with two operands is not worth it to combine.

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