[llvm] [AMDGPU] Add amdgpu-lower-exec-sync pass to lower named-barrier globals (PR #165692)

[Sameer Sahasrabuddhe via llvm-commits]

ssahasra wrote:

> For any pass that will be newly introduced in the amdgpu backend, developers usually follow the approach of two PRs
> 
>     1. Creating the pass, with both legacy and new pass manager implementations.
> 
>     2. Adding the pass into the `amdgpu` pipeline.
> 
> 
> Have followed the same for this change as well. Additionally, removal of changes from `lower-module-lds` were made into separate PR as per feedback from @shiltian

This is not a new pass. It's _existing code_  bundled into a pass as a refactoring. I still think this should be a single change from the point of view of someone looking at the git log. All failures should be traceable to this one transaction.

https://github.com/llvm/llvm-project/pull/165692