[llvm] [AArch64][GlobalISel] Prevented llvm.aarch64.neon.saddlp/uaddlp fallback (PR #160883)

[Joshua Rodriguez via llvm-commits]

JoshdRod wrote:

> Can you add a MIR regbank test as well? Normally I don't require one but I think it's useful in this case.

Test added. Let me know if there are any problems :+1: 

https://github.com/llvm/llvm-project/pull/160883