Virtual instructions

Quoting the Jolt docs:

Some instructions, like division, don't neatly adhere to the lookup table structure required by prefix-suffix Shout. To handle these cases, the problematic instruction is expanded into a virtual sequence, a series of instructions (some potentially virtual).

Most opcodes in ONNX are non-linear, which makes it difficult to provide the structure required by Shout. Thus, we convert an instruction into multiple virtual instructions that behave as one.