Tested with a more reasonable setup: Linux 4.20-rc1, radv 18.3-rc2, LLVM 7.0.0.
The GPU stang still happens.
If it helps rule out anything: This patch does not help https://patchwork.freedesktop.org/patch/256048/
It works now on latest mesa git + llvm git.

Possible fix (not tested if it really was it): https://cgit.freedesktop.org/mesa/mesa/commit/?id=08ea6b9d9bb047603c249468dfe00d7bb9603d5e
Nothing extracted yet.
Your system information
I noticed this in one Unity windows application run with proton that has been working fine on an RX 480, but caused a GPU hang on a Vega 64. The GPU hang does not happen on startup, there's first a simple scenario, but when loading another scenario, it immediately hangs.
I found another application that is similar: Waltz of the Wizard. The first tutorial room works, then you open a door, it renders one frame of the next scenario, and the GPU hangs. I have not tried that one on RX 480 but I assume it would work fine, too.
Only tested with latest mesa git and latest amdgpu development code from drm-next-4.21-wip, I will have to test with a more stable kernel later.