[llvm] [LoopVectorize] Enable vectorisation of early exit loops with live-outs (PR #120567)

[David Sherwood via llvm-commits]

================
@@ -501,8 +501,15 @@ void VPBasicBlock::execute(VPTransformState *State) {
     UnreachableInst *Terminator = State->Builder.CreateUnreachable();
     // Register NewBB in its loop. In innermost loops its the same for all
     // BB's.
-    if (State->CurrentParentLoop)
-      State->CurrentParentLoop->addBasicBlockToLoop(NewBB, *State->LI);
+    Loop *ParentLoop = State->CurrentParentLoop;
----------------
david-arm wrote:

It's possible, although you'll still end up with similar looking code in VPlan::execute where we execute the blocks:

```
  for (VPBlockBase *Block : RPOT)
    Block->execute(State);
```

It's worth bearing in mind that in VPRegionBlock::execute we also set the ParentLoop:

```
  if (!isReplicator()) {
    // Create and register the new vector loop.
    Loop *PrevLoop = State->CurrentParentLoop;
    State->CurrentParentLoop = State->LI->AllocateLoop();
```

and VPRegionBlock is included in that list. So depending upon how the blocks are traversed you may need to cache the previous value each time, i.e. something like:

```
  for (VPBlockBase *Block : RPOT) {
    Loop *OldParentLoop = State->ParentLoop;
    if (Block->getSingleSuccessor() && isExitBlock(Block->getSingleSuccessor()))
      State->ParentLoop = ... parent for exit ...
    Block->execute(State);
    State->ParentLoop = OldParentLoop;
  }
```

I'm honestly not sure if that's any better? The only way to do this neatly I think is if you deal with the VP blocks with a single successor exit block in a different loop, i.e.

```
  for (VPBlockBase *Block : RPOT) {
    if (!Block->getSingleSuccessor() || !isExitBlock(Block->getSingleSuccessor()))
      Block->execute(State);
  }
```

```
  for (VPBlockBase *Block : RPOT) {
    if (Block->getSingleSuccessor() && isExitBlock(Block->getSingleSuccessor())) {
      State->ParentLoop = ... exit block parent loop ...
      Block->execute(State);
    }
  }
```

https://github.com/llvm/llvm-project/pull/120567