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.
What do you mean by inaccurate? It shouldn't change the output to send non-denormals through the slow path (unless they're NANs).