AMDGPU/GlobalISel: add AMDGPUGlobalISelDivergenceLowering pass (#75340)

Add empty AMDGPUGlobalISelDivergenceLowering pass. This pass will
implement
- selection of divergent i1 phis as lane mask phis, requires lane mask
merging in some cases
- lower uses of divergent i1 values outside of the cycle using lane mask
merging
- lowering of all cases of temporal divergence:
- lower uses of uniform i1 values outside of the cycle using lane mask
merging
- lower uses of uniform non-i1 values outside of the cycle using a copy
to vgpr inside of the cycle

Add very detailed set of regression tests for cases mentioned above.

patch 1 from: https://github.com/llvm/llvm-project/pull/73337