Updated: It has been merged into Mesa, just before I clicked publish on this—what timing! From what they said, it should be available in Mesa 19.3 and it can be enabled with the "RADV_PERFTEST=aco" environment variable (source).
Back in early July, Valve announced their work on a new AMD GPU shader compiler for Mesa named ACO and now they're trying to get it pulled into Mesa directly.
Their main aims with ACO were to get the "best-possible code generation for game shaders, and fastest-possible compilation speed" and to replace the currently used shader compiler from the massive LLVM project. It has certainly seemed promising, improving both shader compile time resulting in less stuttering and so helping to improve overall FPS and smoothness in Linux games when played on supported AMD GPUs.
Just this week, a merge request was opened to get ACO into Mesa officially and so far the reception does seem quite positive. It's not in yet though and it may need more work doing and adjustments before it's actually accepted. It's likely far too late for Mesa 19.2 which is due to release very soon, so hopefully Mesa 19.3 currently due towards the end of the year will see it.
You can see Valve's original announcement with more details about ACO here, although with the new Steam display view for news and events it doesn't load for me so use the Wayback Machine if you have the same issue.
To see a little more discussion about it, there's people testing it in our forum.