PowerPC register and memory cost model

[David Edelsohn]

	I compared skidmarks3 performance before and after the proposed
changes to REGISTER_MOVE_COST and MEMORY_MOVE_COST.  Most tests have the
same performance, but 

ParseVid time increases from 166 usecs to 196 usecs
VolInt   time decreases from 147 usecs to 138 usecs

I will retry with FP memory move cost bumped up slightly, as it was in the
earlier model.

David