[llvm] [Debuginfo][TailCallElim] Fix #86262: drop the debug location of entry branch (PR #86269)

[Stephen Tozer via llvm-commits]

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@@ -510,7 +510,11 @@ void TailRecursionEliminator::createTailRecurseLoopHeader(CallInst *CI) {
   NewEntry->takeName(HeaderBB);
   HeaderBB->setName("tailrecurse");
   BranchInst *BI = BranchInst::Create(HeaderBB, NewEntry);
-  BI->setDebugLoc(CI->getDebugLoc());
+  // If the new branch preserves the debug location of CI, it could result in
+  // misleading stepping, if CI is located in a conditional branch.
+  // So, here we don't give any debug location to BI, and use dropLocation()
+  // to explicitly present this dicision.
+  BI->dropLocation();
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SLTozer wrote:

```suggestion
  // If the new branch preserves the debug location of CI, it could result in
  // misleading stepping, if CI is located in a conditional branch.
  // So, here we don't give any debug location to BI.
```
I appreciate the code-as-documentation approach, but since this is guaranteed to be a no-op it's probably better to just leave the comment here - that should be explicit enough.

https://github.com/llvm/llvm-project/pull/86269