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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197535 91177308-0d34-0410-b5e6-96231b3b80d8