Bye Bye PPC_OPCODE_VSX3

Bye Bye PPC_OPCODE_VSX3

This bit is also useless as it can be replaced with PPC_OPCODE_POWER9.
Defining the VSX2 and VSX3 selection based on cpu bits also lets the
assembler/disassembler distinguish between the power7 VSX opcodes and
the power8 ones.  Note that this change means -mvsx now reverts back
to just adding the power7 VSX insns.

include/
	* opcode/ppc.h (PPC_OPCODE_VSX3): Delete.
opcodes/
	* ppc-dis.c (ppc_opts): Remove PPC_OPCODE_VSX3.
	* ppc-opc.c (PPCVSX2): Define as PPC_OPCODE_POWER8.
	(PPCVSX3): Define as PPC_OPCODE_POWER9.