[llvm] [AMDGPU] Introduce conditional barrier pseudo instruction (PR #171604)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> How is a frontend expected to generate this? I'd expect to see a definition for a new intrinsic.

In general the backend shouldn't be in the business of providing high level operations that don't map to an instruction either 

https://github.com/llvm/llvm-project/pull/171604