[PowerPC] Add fast-isel branch and compare selection.

First chunk of actual fast-isel selection code.  This handles direct
and indirect branches, as well as feeding compares for direct
branches.  PPCFastISel::PPCEmitIntExt() is just roughed in and will be
expanded in a future patch.  This also corrects a problem with
selection for constant pool entries in JIT mode or with small code
model.