[clang] [CIR][AArch64] add vshr_* builtins (PR #186693)

[Andrzej Warzyński via cfe-commits]

banach-space wrote:

> I have not included switching to `cir.vec.splat` yet, however after noticing @andykaylor thinks this is the path forward, I tried it locally with this change and seem to be getting correct CIR. 

Thanks for checking!

> Let me know if you want me to modify the code.

Yes, please!


https://github.com/llvm/llvm-project/pull/186693