[llvm] [IR] Avoid self-referencing values caused by PHI node removal (PR #129501)

[Robert Imschweiler via llvm-commits]

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@@ -381,6 +385,26 @@ bool JumpThreadingPass::runImpl(Function &F_, FunctionAnalysisManager *FAM_,
   return EverChanged;
 }
 
+SmallVector<BasicBlock *, 32>
+JumpThreadingPass::unreachableFromBB(BasicBlock *BB, DomTreeUpdater *DTU) {
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ro-i wrote:

Unfortunately, it's not enough to enhance the two unreachable checks here:
```c++
do {
    Changed = false;
    for (auto &BB : *F) {
      if (Unreachable.count(&BB))
        continue;
      while (processBlock(&BB)) // Thread all of the branches we can over BB.
```
and here
```c++
      if (pred_empty(&BB)) {
        // When processBlock makes BB unreachable it doesn't bother to fix up
        // the instructions in it. We must remove BB to prevent invalid IR.
```
with `!DT.isReachableFromEntry`. Since the domtree is (mostly?) lazily updated during the pass, my own test case which I added to unreachable-loops.ll still goes into recursion.
Or are you thinking about something else?

https://github.com/llvm/llvm-project/pull/129501