Flush vfpu prefixes, initial implementation of VecDo3

This implements a dirty flag for prefixes and conservatively assumes a `Comp_Generic()` with `IS_VFPU` might or might not eat prefixes (therefore they are unknown after the call.)

It also implements a probably-not-very-fast VecDo3, and makes it and VDot bail if prefixes aren't known or if they have some effect.

So far it works (it didn't when I was accidentally using `.R()`, heh), but I'm planning to write something to generate tests for each of these.  I left them both disabled for now in case there are issues.

-[Unknown]