[DebugInfo][RemoveDIs] Cope with instructions moving after themselves (#74113)

We occasionally move instructions to their own positions, which is not
an error, and has no effect on the program. However, if dbg.value
intrinsics are present then they can effectively be moved without any
other effect on the program.

This is a problem if we're using non-instruction debug-info: a moveAfter
that appears to be a no-op in RemoveDIs mode can jump in front of
intrinsics in dbg.value mode. Thus: if an instruction is moved to itself
and the head bit is set, force attached debug-info to shift down one
instruction, which replicates the dbg.value behaviour.

1 files changed, 3 insertions, 2 deletions

diff --git a/llvm/lib/IR/Instruction.cpp b/llvm/lib/IR/Instruction.cpp
index a6e43de..920a2ec 100644
--- a/llvm/lib/IR/Instruction.cpp
+++ b/llvm/lib/IR/Instruction.cpp
@@ -200,8 +200,9 @@ void Instruction::moveBeforeImpl(BasicBlock &BB, InstListType::iterator I,
   // If we've been given the "Preserve" flag, then just move the DPValues with
   // the instruction, no more special handling needed.
   if (BB.IsNewDbgInfoFormat && DbgMarker && !Preserve) {
-    if (I != this->getIterator()) {
-      // "this" is definitely moving; detach any existing DPValues.
+    if (I != this->getIterator() || InsertAtHead) {
+      // "this" is definitely moving in the list, or it's moving ahead of its
+      // attached DPValues. Detach any existing DPValues.
       handleMarkerRemoval();
     }
   }