x86: Rename VexOpcode to OpcodePrefix

x86: Rename VexOpcode to OpcodePrefix

Rename VexOpcode to OpcodePrefix so that OpcodePrefix can be used for
regular encoding prefix.

gas/

	* config/tc-i386.c (build_vex_prefix): Replace vexopcode with
	opcodeprefix.
	(build_evex_prefix): Likewise.
	(is_any_vex_encoding): Don't check vexopcode.
	(output_insn): Handle opcodeprefix.

opcodes/

	* i386-gen.c (opcode_modifiers): Replace VexOpcode with
	OpcodePrefix.
	* i386-opc.h (VexOpcode): Renamed to ...
	(OpcodePrefix): This.
	(PREFIX_NONE): New.
	(PREFIX_0X66): Likewise.
	(PREFIX_0XF2): Likewise.
	(PREFIX_0XF3): Likewise.
	* i386-opc.tbl (Prefix_0X66): New.
	(Prefix_0XF2): Likewise.
	(Prefix_0XF3): Likewise.
	Replace VexOpcode= with OpcodePrefix=.  Use Prefix_0X66 on xorpd.
	Use Prefix_0XF3 on cvtdq2pd.  Use Prefix_0XF2 on cvtpd2dq.
	* i386-tbl.h: Regenerated.