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

[Matt Arsenault via llvm-commits]

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arsenm wrote:

We shouldn't really need this special case. We don't really need amdgcn_unreachable. I thought it's only inserted by SIAnnotateControlFlow?

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