Intrinsics would be a great quality of life improvement for low-level optimisations. They don't require understanding register allocation, but obviously they would add complexity to the compiler and they aren't cross-architecture. I have tried some tools that convert a C function with intrinsics to Go assembly, but they were buggy for my use case [1],[2].

[1] github.com/minio/c2goasm (no longer updated)

[2] https://github.com/gorse-io/goat