[llvm] [DebugInfo][SimpleLoopUnswitch] Fix missing debug location updates (PR #97662)

[Shan Huang via llvm-commits]

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@@ -2334,21 +2338,25 @@ static void unswitchNontrivialInvariants(
   // nuke the initial terminator placed in the split block.
   SplitBB->getTerminator()->eraseFromParent();
   if (FullUnswitch) {
-    // Splice the terminator from the original loop and rewrite its
-    // successors.
-    TI.moveBefore(*SplitBB, SplitBB->end());
-
     // Keep a clone of the terminator for MSSA updates.
     Instruction *NewTI = TI.clone();
     NewTI->insertInto(ParentBB, ParentBB->end());
 
+    // Splice the terminator from the original loop and rewrite its
+    // successors.
+    TI.moveBefore(*SplitBB, SplitBB->end());
+    TI.dropLocation();
----------------
Apochens wrote:

`TI` is moved out of the loop and its debug location should be dropped. However, `NewTI` (the code moved up) which is cloned from `TI` should preserve `TI`'s debugl location, because `NewTI` is inserted into the original place where `TI` is at. In addition, `NewTI` will be replaced by a new branch instruction, which should inherit the debug location of `NewTI`.

So, I move the code down after the clone to preserve the debug location so that `NewTI` can preserve the right debug location while the debug location of `TI` can be correctly dropped.

https://github.com/llvm/llvm-project/pull/97662