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

[Pierre van Houtryve via llvm-commits]

https://github.com/Pierre-vh commented:

Some more docs in `AMDGPUGlobalISelDivergenceLowering.cpp` would be nice.
Seems like it's going to be a core part of the GlobalISel pipeline so IMO it should be well documented so it's not too hard to understand and contribute to.

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