x86: Use vexvvvv as the switch state to encode the vvvv register

Use vexvvvv as the switch state, and replace VexVVVV with Src1VVVV.
Src1VVVV means using VEX.vvvv encodes the first source register
operand. The old logic did not check vexvvvv first, which made the
logic here very complicated.

gas/ChangeLog:

        * config/tc-i386.c (optimize_encoding): Replaced 1 with Src1VVVV.
        (build_modrm_byte): Used vexvvvv to encode the vvvv register.
        (s_insn): Replaced 1 with Src1VVVV.

opcodes/ChangeLog:

        * i386-opc.h (VexVVVV_DST): Adjusted the value.
        (Src1VVVV): New.
        * i386-opc.tbl: Replaced part VexVVVV with Src1VVVV.
	* i386-tbl.h: Regenerated.