[llvm] [AMDGPU] Insert readfirstlane in the function returns in sgpr. (PR #135326)

[via llvm-commits]

dyung wrote:

> This broke a couple of our bots. E.g. https://lab.llvm.org/buildbot/#/builders/10/builds/3849
> 
> They were red due to prior fails, so did not comment here. Edit:
> 
> ```
> FAIL: LLVM::sdag-print-divergence.ll
> ```

This test is also failing on two of my bots as well:
- https://lab.llvm.org/buildbot/#/builders/174/builds/16532
- https://lab.llvm.org/buildbot/#/builders/208/builds/542

Can you please take a look and revert if you need time to investigate?

https://github.com/llvm/llvm-project/pull/135326