x86: Add {vex} pseudo prefix

There are 2-byte VEX prefix and 3-byte VEX prefix.  2-byte VEX prefix
can't encode all operands.  By default, assembler tries 2-byte VEX prefix
first.  {vex3} can be used to force 3-byte VEX prefix.  This patch adds
{vex} pseudo prefix and keeps {vex2} for backward compatibility.

gas/

	* config/tc-i386.c (_i386_insn): Replace vex_encoding_vex2
	with vex_encoding_vex.
	(parse_insn): Likewise.
	* doc/c-i386.texi: Replace {vex2} with {vex}.  Update {vex}
	and {vex3} documentation.
	* testsuite/gas/i386/pseudos.s: Replace 3 {vex2} tests with
	{vex}.
	* testsuite/gas/i386/x86-64-pseudos.s: Likewise.

opcodes/

	* i386-opc.tbl: Add {vex} pseudo prefix.
	* i386-tbl.h: Regenerated.