When vectorized, the CR inputs/outputs are read/written to 4-bit CR fields
starting from CR6 and incrementing from there. If CR63 is reached, the next CR
field used wraps around to CR0, then incrementing from there.
+(see [[discussion]]. an alternative scheme is described there)
CR6 was chosen to balance avoiding needing to save CR2-CR4 (which are
callee-saved) just to use SV vectors with VL <= 61 as well as having the first