126.96.36.199: call an appropriate existing ctor from MAKE-INSTANCE (CLASS)
The intuition here is that searching for a ctor is much cheaper than
the whole of the unoptimized make-instance machinery. If a class has
many, many ctors, this may not be true, but that should be the rare
Only call optimized ctors so that we don't risk endless recursion
through the fallback case.