[llvm] Late temporal divergence lowering for SDAG (PR #67033)

[via llvm-commits]

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ruiling wrote:

We can also try to remove some unnecessary blocks but still show the problem. This might be easy to get an LLVM IR based lit-test. But the separate pass might need some hand-crafted MachineIR test to cover more cases.

https://github.com/llvm/llvm-project/pull/67033