[llvm] [DFAJumpThreading] Avoid exploring the paths that never come back (PR #85505)

[via llvm-commits]

XChy wrote:

> Can you please add something like `if (VerifyLoopInfo) LI->verify(DT)` and add `-verify-loop-info` to all the DFAJumpThreading tests? Can also mark the analysis as preserved.

I don't think we preserve the loop analysis completely. When transforming DFA, we clone and thread BBs, which may break loop analysis. This patch only preserves it before transforming DFA, so that we get the correct result in `AllSwitchPaths::paths`.

https://github.com/llvm/llvm-project/pull/85505