bit format (single instruction option) or a variable
length VLIW-like prefix (multi or "grouped" option).
* The prefix(es) indicate which registers are "tagged" as
- "vectorised". Predicates can also be added.
+ "vectorised". Predicates can also be added, and element widths overridden on any src or dest register.
* A "Vector Length" CSR is set, indicating the span of any future
"parallel" operations.
* If any operation (a **scalar** standard RV opcode) uses a register