Optimizations to many vops to allow a descriptor for the second arg.

 * Allow the second arg to be a descriptor for the basic arithmetic
   operations on single floats and double floats.  Previously, the
   boxed number would be loaded to a temporary reg for the
   operation. This saves the load and the extra register.

 * Allow a descriptor for the second arg in the comparison vops, the
   float conversion vops and the float truncation vops.

 * Allow descriptor for sqrt vop.