i386: Improve index_register_operand predicate

i386: Improve index_register_operand predicate

Use the same approach as in register_no_elim_operand predicate, but also
reject stack_pointer_rtx operands.

gcc/ChangeLog:

	* config/i386/predicates.md (index_register_operand): Reject
	arg_pointer_rtx, frame_pointer_rtx, stack_pointer_rtx and
	VIRTUAL_REGISTER_P operands.  Allow subregs of memory before reload.
	(call_register_no_elim_operand): Rewrite as ...
	(call_register_operand): ... this.
	(call_insn_operand): Use call_register_operand predicate.