s390: Add test cases for base/index register 0

s390: Add test cases for base/index register 0

While at it add comments to logic to omit base and/or index register 0
in s390 disassembly.

opcodes/
	* s390-dis.c: Add comments related to omitting base and/or index
	register 0 in disassembly.
gas/
	* testsuite/gas/s390/s390.exp: Add test cases for base and/or
	index register 0.
	* testsuite/gas/s390/zarch-base-index-0.s: Add test cases for
	base and/or index register 0.
	* testsuite/gas/s390/zarch-base-index-0.d: Likewise.
	* testsuite/gas/s390/zarch-base-index-0-err.s: Add error test
	cases for base and/or index register 0.

Reviewed-by: Andreas Krebbel <krebbel@linux.ibm.com>
Signed-off-by: Jens Remus <jremus@linux.ibm.com>