Implement vsbn.s and vwbn.s

Both match tests and appear to work properly.  I still won't pretend to understand exactly what vwbn.s is doing... but I think I kinda understand it.  The `1 << (23 - shift)` is for the implicit leading bit of the mantissa.... actually, I should simplify that.  I'm just not really sure how 0xF is relevant here.

Did not actually test prefixes.

-[Unknown]