[llvm] Revert "[SimplifyCFG] Avoid threading for loop headers" (PR #155533)

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

This patch triggers a bug in vector-combine. But I don't think it should be reverted if it doesn't break the CI.
I have filed https://github.com/llvm/llvm-project/issues/155543 to track the issue. If no one works on it, I will post a fix tonight (UTC+8).

https://github.com/llvm/llvm-project/pull/155533