[llvm] [AArch64] Range-check the ext byte index (PR #214468)

[Zane Hambly via llvm-commits]

Zaneham wrote:

I'm closing this because I was so "lost in it" that I didn't see the other PRs posted the other day. I've had a read of their implementation and will give my test findings. 

https://github.com/llvm/llvm-project/pull/214468