s390: Simplify parsing of omitted index register operand

The index register operand X in D(X,B) can optionally be omitted by
coding D(,B) or D(B).  Simplify the parsing logic.

gas/
	* config/tc-s390.c (md_gather_operands): Rename
	omitted_base_or_index to omitted_index and simplify logic.

Signed-off-by: Jens Remus <jremus@linux.ibm.com>