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

[Petar Avramovic via llvm-commits]

petar-avramovic wrote:

> 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.

Each individual patch passes check-llvm, ir is correct after the last patch but mir updates are relevant and give good overview of the changes (PHIs go from partially selected to fully selected). I wanted to open separate pr for each of the patches if they are ok together (don't know if there is a better way).

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