[llvm] AMDGPU: Add sgpr bit convert tests (PR #136112)

[Shilei Tian via llvm-commits]

shiltian wrote:

> > Hi @mikaelholmen , the fix up PR has been merges, please check. Sorry again and thank you for notifying me!
> 
> Great!
> 
> I'm a little surprised that there are no comments here in this patch from failing expensive-checks build bots due to this. Now I saw the failure in downstream non-public bots instead.

It exposes some bugs in the existing code.

https://github.com/llvm/llvm-project/pull/136112