at a time, standard single (v3.0B) CTR decrementing should
correspondingly be used.
+If CTR+VLSET Modes are requested, the amount that CTR is decremented
+by is the value of VL *after* truncation (should that occur).
+
Note that, interestingly, due to the useful side-effects of `VLSET` mode
it is actually useful to use Branch Conditional even
to perform no actual branch operation, i.e to point to the instruction