[llvm] [AMDGPU] Add islands to StructurizeCFG (PR #201675)

[Robert Imschweiler via llvm-commits]

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ro-i wrote:

I also don't like the linear scan for amdgcn_unreachable, but the block size is expected to be tiny and I didn't want to hardcode the exact situation that is constructed by AMDGPUUnifyDivergentExitNodes

https://github.com/llvm/llvm-project/pull/201675