[PowerPC] Pre-commit FISel with PC-Rel test

Our handling of PC-Relative addressing is currently broken with
Fast ISel in 3 ways:
- FISel emits calls without handling all the PC-Rel intricacies
- FISel materializes FP constants through the TOC
- FISel materializes GV's through the TOC

As it would be unnecessarily tedious to implement all the handling
for PC-Rel in Fast ISel, we will turn off FISel for anything that
generates references to the TOC.