[llvm] [AArch64] Remove superfluous sxtw in peephole opt (PR #96293)

[David Green via llvm-commits]

davemgreen wrote:

Thanks - the reproducer in the commit message was useful for figuring out what was wrong. It just needs to make sure it removed all the copies. I'll put together a recommit - please let me know if you run into any more problems, and we can revert again to make sure the branch is clean.

https://github.com/llvm/llvm-project/pull/96293