[llvm] [AMDGPU][GlobalISel] Avoid creating dead S1 truncs (PR #68788)

[Pierre van Houtryve via llvm-commits]

Pierre-vh wrote:

Now that I think about it, I think we should be cleaning them up. Can dead instructions ever reach ISel? Where/when do we delete them?

https://github.com/llvm/llvm-project/pull/68788