[llvm] [AMDGPU] Fix v_mov_b16 pair merging when the second mov reads the first (PR #227502)

[Krzysztof Drewniak via llvm-commits]

https://github.com/krzysz00 approved this pull request.

Thanks for the end-to-end reduction, approved!

https://github.com/llvm/llvm-project/pull/227502