[llvm] [JumpThreading] Remove deleted BB from Unreachable (PR #126984)

[Nikita Popov via llvm-commits]

nikic wrote:

> As you have noticed MergeBasicBlockIntoOnlyPred can mark an unreachable block for deletion. The deletion is deferred until DTU->flush() is called, then the unreachable BB is actually deleted, but the set Unreachable is not modified, so a deleted BB pointer is left in Unreachable.

I think the last bit I'm still missing is when JumpThreading calls DTU->flush() *during* the transform. I thought JT only flushes DT updates after everything is done, at which point the Unreachable set is not relevant anymore.

https://github.com/llvm/llvm-project/pull/126984