this way SV may be entirely disabled, leaving an "all zeros" to indicate to v3.1B 64bit prefixing that the standard OpenPOWER v3.1B encodings are in full effect (and that SV is not). As all zeros meshes with current "reserved" encodings this should work well.
+(but not disable use of VL or anything related to SVSTATE, that is
+completely different. above refers to instruction **augmentation**)
## twin predication