[llvm] [LFI][AArch64] Fix Segfault due to infinite recursion in LFI rewriter (PR #210267)

[Zachary Yedidia via llvm-commits]

zyedidia wrote:

Thanks. I think rather than relying on ordering between setting `DeferredLRGuard` and `emitAddMask` you can just add

```
if (Guard)
  return;
```

To the beginning of `onLabel` to ensure that any labels emitted by the rewriter itself aren't processed further by `onLabel`. `Guard` is already meant as a recursion guard, so this is the right use-case for that variable.

https://github.com/llvm/llvm-project/pull/210267