[llvm] [AMDGPU] Fix true16 v2s copy of a 16-bit subreg losing its source (PR #225175)

[Zach Goldthorpe via llvm-commits]

zGoldthorpe wrote:

> > IIRC there is another PR that is trying to fix the same thing from @zGoldthorpe ?
> 
> Right, this is to fix a regression caused by https://github.com/llvm/llvm-project/pull/211848

Is the issue also fixed by #22175?

https://github.com/llvm/llvm-project/pull/225175