[llvm] [SimplifyCFG]divergent undef predecessor removal (PR #204287)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> I think it's too late to do anything once we're in SimplifyCFG. If you're not happy with the "noundef" marking on the arguments to `__shfl_sync`, that seems like something we should handle in the frontend.

We do have a really hacky attribute specifically for this case, `__attribute__((maybe_undef))` added in a35c64ce23b7

https://github.com/llvm/llvm-project/pull/204287