[clang] [llvm] [mlir] [OMPIRBuilder] always leave PARALLEL via the same barrier (PR #164586)

[Michael Kruse via cfe-commits]

================
@@ -1800,8 +1779,18 @@ OpenMPIRBuilder::InsertPointOrErrorTy OpenMPIRBuilder::createParallel(
   Instruction *PRegPreFiniTI = PRegPreFiniBB->getTerminator();
 
   InsertPointTy PreFiniIP(PRegPreFiniBB, PRegPreFiniTI->getIterator());
-  if (Error Err = FiniCB(PreFiniIP))
-    return Err;
+  if (!FiniInfo.FiniBB) {
+    if (Error Err = FiniCB(PreFiniIP))
----------------
Meinersbur wrote:

Can we improve the situation a bit to make it more predictable? There seem to be two code locations where FiniCB is called and FiniBB is assigned, here or in `commonDirectiveExit`. I propose refactoring this out into a different function whose task it is to get the FiniBB: If it was already created, return it; otherwise emit one. Such as

```cpp
struct FinalizationInfo {
  ...
  BasicBlock *getOrCreateFinializationBranchTarget() {
    if (!FiniBB) {
     FiniCB(FiniIP);
     FiniBB = FinIP.getBlock();
    }
    return FiniBB;
  }
};
```

https://github.com/llvm/llvm-project/pull/164586