Eliminate PPC instruction decoding ambiguities

The instruction definitions in the PPC backend have a number of variants
defined for the same instruction to represent differences between 64-bit and
32-bit semantics. In order to generate a disassembler for the PPC backend, we
need to mark all but one of these as CodeGen only.

No functionality change intended; this is prep work for PPC disassembly
support.

llvm-svn: 197535