[llvm] [GlobalISel] Turn shuffle a, b, mask -> shuffle undef, b, mask iff mask does not reference a (PR #115377)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

To clean up CombinerHelper, you also move the combine to:
https://github.com/llvm/llvm-project/blob/main/llvm/lib/CodeGen/GlobalISel/CombinerHelperVectorOps.cpp

https://github.com/llvm/llvm-project/pull/115377