[PowerPC] More fast-isel chunks (returns and integer extends)

Incremental improvement to fast-isel for PPC64.  This allows us to
select on ret, sext, and zext.  Filling in sext/zext improves some of
the existing logic in handling compare-immediates that needed extends.

A simplified return convention for fast-isel is also added to the
PPC64 calling conventions.  All call/return processing for DAG
selection is handled with custom code, so there isn't an existing CC
to rely on here.  The include of PPCGenCallingConv.inc causes compiler
warnings due to the 32-bit calling conventions that are not used, so
the dummy function "usePPC32CCs()" is added here to silence those.

Test cases for the return and extend logic are added.