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

[via llvm-commits]

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@@ -575,6 +582,12 @@ struct AllSwitchPaths {
 
     Visited.insert(BB);
 
+    // Stop if we have reached the BB out of loop, since its successors have no
+    // impact on the DFA.
+    // TODO: Do we need to stop exploring if BB is the outer loop of the switch?
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XChy wrote:

Yes, it would break @test2 in dfa-jump-threading-transform.ll. This test contains nested loops. If we don't see the outer loop, we can only thread part of switch.

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