Bye bye PPC_OPCODE_ALTIVEC2

Bye bye PPC_OPCODE_ALTIVEC2

This bit is worse than useless.  Using it prevents the assembler and
disassembler distinguishing between opcodes added for power8 and those
added for power9.

include/
	* opcode/ppc.h (PPC_OPCODE_ALTIVEC2): Delete.
opcodes/
	* ppc-dis.c (ppc_opts): Remove PPC_OPCODE_ALTIVEC2.
	* ppc-opc.c (PPCVEC2): Define as PPC_OPCODE_POWER8|PPC_OPCODE_E6500.
	(PPCVEC3): Define as PPC_OPCODE_POWER9.