i386: Check invalid (%dx) usage

i386: Check invalid (%dx) usage

(%dx) isn't a valid memory address in any modes.  It is used as a special
memory operand for input/output port address in AT&T syntax and should
only be used with input/output instructions.  Update i386_att_operand to
set i.input_output_operand to true for (%dx) and issue an error if (%dx)
is used with non-input/output instructions.

	PR gas/29751
	* config/tc-i386.c (_i386_insn): Add input_output_operand.
	(md_assemble): Issue an error if input/output memory operand is
	used with non-input/output instructions.
	(i386_att_operand): Set i.input_output_operand to true for
	(%dx).
	* testsuite/gas/i386/inval.l: Updated.
	* testsuite/gas/i386/x86-64-inval.l: Likewise.
	* testsuite/gas/i386/inval.s: Add tests for invalid (%dx) usage.
	* testsuite/gas/i386/x86-64-inval.s: Likewise.