[llvm] [AArch64][GlobalISel] Add custom legalization for v4s8 = G_TRUNC v4s16 (PR #85610)

[David Green via llvm-commits]

davemgreen wrote:

I've put up #85625 that shows what it does. I'm not sure which is better in the long run between that method and this one, but I think it would be better to add support for the other types if we do use bitcasts.

https://github.com/llvm/llvm-project/pull/85610