[llvm] [OMPIRBuilder] Avoid crash in BasicBlock::splice. (PR #154987)

[Michael Kruse via llvm-commits]

================
@@ -307,7 +307,14 @@ void llvm::spliceBB(IRBuilderBase::InsertPoint IP, BasicBlock *New,
 
   // Move instructions to new block.
   BasicBlock *Old = IP.getBlock();
-  if (!Old->empty() || !New->empty())
+  // If the Old block is empty then there are no instructions to move. But in
+  // the new debug scheme, it could have trailing debug records which will be
+  // moved to New in spliceDebugInfoEmptyBlock. We dont want that for 2 reasons:
+  // 1. If New is also empty, it could cause a crash.
+  // 2. Even if New is not empty, we want to keep those debug records in the
+  // Old as that was the behavior with the old scheme (debug intrinsics).
----------------
Meinersbur wrote:

```suggestion
  // 2. Even if New is not empty, we want to keep those debug records in the
  // Old as that was the behavior with the old scheme (debug intrinsics).
```
[nit] Just being the legacy scheme doesn't mean it is more correct. Consider something that mentions the rationale from `BasicBlock::spliceDebugInfoEmptyBlock`:
```
  // If the source block is completely empty, including no terminator, then
  // transfer any trailing DbgRecords that are still hanging around. This can
  // occur when a block is optimised away and the terminator has been moved
  // somewhere else.
```
which does not apply here since we still want to use `Old` BB (like adding a BrInst with `CreateBranch`).

https://github.com/llvm/llvm-project/pull/154987