SVP64 has the means to mark registers as scalar or vector. However
the available space in the prefix is extremely limited (9 bits).
With effectively 5 operands (3 in, 2 out) some compromises are needed.
-A little though gives a useful workaround: two modes,
+A little thought gives a useful workaround: two modes,
controlled by a single bit in `RM.EXTRA`, determine whether the 5th
register is set to RC or whether to RT+VL. This then leaves only
4 registers to qualify as scalar/vector, which can use four