[llvm] GlobalISel lane masks merging (PR #73337)

[Nicolai Hähnle via llvm-commits]

https://github.com/nhaehnle commented:

It's probably for the best to land this PR soon. I have a few detail comments still inline, but hopefully they're not too complex.

I am somewhat surprised about the order in which the changes appear. Though you did update the tests, so did you actually run a check (e.g. using `git rebase -x "ninja -C build check-llvm-codegen-amdgpu" ...) that passes after each individual commit? If yes, then great. That would make me feel a bit more confident about the structure of this.

https://github.com/llvm/llvm-project/pull/73337