Clean base_reg and assign correct values to regs for input_output_operand (%dx).

Clean base_reg and assign correct values to regs for input_output_operand (%dx).

For special processing of input and output operands (%dx),
the state of some variables needs to be cleaned.

gas/ChangeLog:

	* config/tc-i386.c (i386_att_operand): Assign values to regs and
	clean i.base_reg for input output operand (%dx).