[llvm] [AArch64][ISel] Add custom lowering for clmul nxv8i16 (PR #195893)

[Paul Walker via llvm-commits]

paulwalker-arm wrote:

I've only given this a passing glance so please take with a pinch of salt, but isn't this just `trn1(pmullb(a,b), pmullt(a,b)`?

https://github.com/llvm/llvm-project/pull/195893