S/390: Fix optional operand handling after memory addresses

S/390: Fix optional operand handling after memory addresses

Instructions having an optional argument following a memory address
operand were not handled correctly if the optional argument was not
specified.

gas/ChangeLog:

2018-11-09  Andreas Krebbel  <krebbel@linux.ibm.com>

	* config/tc-s390.c (skip_optargs_p): New function.
	(md_gather_operands): Use skip_optargs_p.
	* testsuite/gas/s390/s390.exp: Run the new test.
	* testsuite/gas/s390/zarch-optargs.d: New test.
	* testsuite/gas/s390/zarch-optargs.s: New test.