[llvm] [LFI][AArch64] Account for deferred LR guard flush in getInstSizeInBytes (PR #224791)

[Zachary Yedidia via llvm-commits]

https://github.com/zyedidia approved this pull request.

LGTM. Since we are moving the main body of `getInstSizeInBytes` to `getInstSizeInBytesImpl` it might be worth a glance from an Arm backend maintainer?

https://github.com/llvm/llvm-project/pull/224791