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.
Bits 0-7 are AL, and bit 8 is the sign bit, so the sign bit sort of gets left by the wayside after the shift. This trickiness avoids an extra "and" to mask out the sign bit.