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).
diff --git a/gas/config/tc-i386.c b/gas/config/tc-i386.c
index 4f3864f..3f1b39b 100644
--- a/gas/config/tc-i386.c
+++ b/gas/config/tc-i386.c
@@ -13016,6 +13016,8 @@
 	  && !operand_type_check (i.types[this_operand], disp))
 	{
 	  i.types[this_operand] = i.base_reg->reg_type;
+	  i.op[this_operand].regs = i.base_reg;
+	  i.base_reg = NULL;
 	  i.input_output_operand = true;
 	  return 1;
 	}