Changes commited for Nate Begeman:

Use a PowerPC specific prolog epilog inserter to control where spilled
callee save regs are placed on the stack.
Get rid of implicit return address stack slot, save return address reg
(LR) in appropriate slot
Improve code generated for functions that don't have calls or access
globals


Note from Chris: PowerPCPEI will eventually be eliminated, once the
functionality is merged into CodeGen/PrologEpilogInserter.cpp

llvm-svn: 15536