JIT: faster PPC_FP code

The PPC_FP conversion code can be made a lot simpler with the observation
that the only values that need to be sent through the slow x87 path are
denormals.

A whole bunch faster: 708->678 seconds on POV-RAY.
Are you sure? As far as I can tell, ConvertDoubleToSingle passes its output directly to storing into memory and never saves it.