[llvm] [AMDGPU] Remove dubious true16 build_vector pattern (PR #206500)

[Jay Foad via llvm-commits]

jayfoad wrote:

> This pattern produces illegal COPY
> 
> ```
> %0:vgpr_16  = COPY %33.lo16
> %46:vgpr_32 = COPY %0
> ```

It's not currently detected as illegal. How serious are we about actually banning this case?

https://github.com/llvm/llvm-project/pull/206500