Split some PPC itinerary classes

In preparation for adding scheduling definitions for the POWER7, split some PPC
itinerary classes so that the P7's latencies and hazards can be better
described. For the most part, this means differentiating indexed from non-index
pre-increment loads and stores. Also, differentiate single from
double-precision sqrt.

No functionality change intended (except for a more-specific latency for
single-precision sqrt on the A2).