[llvm] [AArch64] Fold vector shifts guarded against oversized amounts into USHL (PR #207628)

[Adam Scott via llvm-commits]

as4230 wrote:

> Sorry for the delay, we have been prioritizing out the release. I think this LGTM now. Thanks

Thanks for the reviews! I don't have commit access yet, so could you merge this when you get the chance? I will open up the shift(select) form which is a small extension of this soon after this merges. 

https://github.com/llvm/llvm-project/pull/207628