i386: Clear vex instead of vex.evex

"vex" has many fields to control how to decode an instruction.  Clear
all fields in "vex" before decoding an instruction to avoid using values
left from the previous instruction.

gas/

	PR gdb/23028
	PR binutils/23025
	* testsuite/gas/i386/prefix.s: Add tests for vcvtpd2dq with
	VEX and EVEX prefixes.
	* testsuite/gas/i386/prefix.d: Updated.

opcodes/

	PR gdb/23028
	PR binutils/23025
	* i386-dis.c (get_valid_dis386): Don't set vex.prefix nor vex.w
	to 0.
	(print_insn): Clear vex instead of vex.evex.

(cherry picked from commit caf0678c)