Start cleanup of PPC register definitions using foreach loops.

No functionality change intended.

This captures the first two cases GPR32/64. For the others, we need
an addition operator (if we have one, I've not yet found it).

Based on a suggestion made by Tom Stellard in the AArch64 review!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173366 91177308-0d34-0410-b5e6-96231b3b80d8