[clang] [llvm] [AMDGPU] Move AMDGPUAttributor earlier than AMDGPULowerKernelAttributes (PR #178028)

[Shilei Tian via cfe-commits]

shiltian wrote:

> > How about running AMDGPULowerKernelAttributes a little bit late?
> 
> What we emit out of that should probably see basic function simplification, it shouldn't be as late as it is now

How about right before `AMDGPULowerKernelAttributes`?

https://github.com/llvm/llvm-project/pull/178028