[llvm] AMDGPU/SILowerI1Copies process phi incomings in specific order (PR #72124)

[Nicolai Hähnle via llvm-commits]

https://github.com/nhaehnle commented:

Surely the test can be simplified? Apart from that and the inline comment, this LGTM

https://github.com/llvm/llvm-project/pull/72124