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

[Amara Emerson via llvm-commits]

aemerson wrote:

I think your approach is fine as long as restricting the combine didn't pessimism too much, which it didn't look like it did.

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