[llvm] [LoopVectorize] Use new getUniqueLatchExitBlock routine (PR #108231)

[Graham Hunter via llvm-commits]

================
@@ -159,6 +159,16 @@ BlockT *LoopBase<BlockT, LoopT>::getUniqueExitBlock() const {
   return getExitBlockHelper(this, true).first;
 }
 
+template <class BlockT, class LoopT>
+BlockT *LoopBase<BlockT, LoopT>::getUniqueLatchExitBlock() const {
+  const BlockT *Latch = getLoopLatch();
+  assert(Latch && "Latch block must exists");
+  SmallVector<BlockT *, 4> ExitBlocks;
----------------
huntergr-arm wrote:

```suggestion
  auto IsExitBlock = [&](BlockT *BB, bool AllowRepeats) -> BlockT * {
    assert(!AllowRepeats && "Unexpected parameter value.");
    return !contains(BB) ? BB : nullptr;
  };
  return find_singleton<BlockT>(children<BlockT *>(Latch), IsExitBlock);
```

Requires changing Latch to be a non-const pointer (since there isn't a version of `children` with const), but if we already have the Latch block there's no need to search for it again.

https://github.com/llvm/llvm-project/pull/108231