[llvm] [AMDGPU] Vectorize more 16 bit shuffles (PR #90648)

[Nuno Lopes via llvm-commits]

nunoplopes wrote:

> > Alive2 complains about this patch:
> 
> This patch can't have broken this as it just changes the cost model, this must have just added test coverage that hits it

Or that, yes. Sorry, I meant there is a new test failing. The test didn't exist before.

https://github.com/llvm/llvm-project/pull/90648