spirv-fuzz has a pass that replaces a boolean with a constant binary, e.g. replacing "true" with "5 > 4". At the moment it will make such a replacement for an OpPhi argument, inserting the instruction that computes the binary right before the OpPhi. That's no good, since it's an illegal place for the instruction.

The short term solution is to ban this from applying to OpPhi instructions; a better solution is to add the new instruction to the appropriate parent block of the OpPhi.