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

[Petar Avramovic via llvm-commits]

petar-avramovic wrote:

> Can the amdgpu-global-isel-risky-select flag be deleted?

Yes, will do it in another patch, from the upcoming patches:
`S1 G_PHIs` should not be selected in instruction-select, instead:
- divergent `S1 G_PHI` is pre-selected in AMDGPUGlobalISelDivergenceLowering (into fully inst-selected PHI) 
- uniform `S1 G_PHI` is lowered into `S32 G_PHI` in AMDGPURegBankSelect.cpp (TODO)




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