Implicit uses of non-register value types places impossible to satisfy
constraints on the legalizer / artifact combiner. These prevent
writing sensible legalize rules for the artifacts without triggering
infinite loops in the legalizer.
The verifier really needs to enforce this, but I'm not sure what the
exact conditions would look like yet.
Is this really still testing what it's supposed to test, given that there's no trunc to 2 x s8 in the output?