s390: Do not erroneously use base operand value for length operand

s390: Do not erroneously use base operand value for length operand

The base register operand B may optionally be omitted in D(B) by coding
D and in D(L,B) by coding D(L). The index register operand X may
optionally be omitted in D(X,B) by coding D(,B) or D(B). Both base and
index register operands may optionally be omitted in D(X,B) by coding D.
In any case the omitted base and/or index register operand value
defaults to zero.

When parsing an erroneously omitted length L operand in D(L,B) by coding
D(,B) the base register operand B was erroneously consumed as length
operand. When using a register name for the base register operand this
was detected and reported as error. But when not using a register name
the base register operand value was erroneously used as length operand
value.

Correct the parsing of an omitted optional base or index register to not
erroneously use the base register operand value as length, when
erroneously omitting the length operand.

While at it rename the variable used to remember whether the base or
index register operand was omitted to enhance code readability.
Additionally add test cases for the optional omission of base and/or
index register operands.

Example assembler source:
	mvc	16(1,%r1),32(%r2)
	mvc	16(1),32(%r2)
	mvc	16(,1),32(%r2)		# undetected syntax error

Disassembly of bad assembly without commit shows the base register
operand value was erroneously used as length operand value:
   0:   d2 00 10 10 20 20       mvc     16(1,%r1),32(%r2)
   6:   d2 00 00 10 20 20       mvc     16(1,%r0),32(%r2)
   c:   d2 00 00 10 20 20       mvc     16(1,%r0),32(%r2)

Assembler messages with commit:
3: Error: operand 1: missing operand

gas/
	* config/tc-s390.c: Correct parsing of omitted base register.
	* testsuite/gas/s390/s390.exp: Add test cases for omitted base
	and/or index register.
	* testsuite/gas/s390/zarch-omitted-base-index.s: Test cases for
	omitted optional base or index register.
	* testsuite/gas/s390/zarch-omitted-base-index.d: Likewise.
	* testsuite/gas/s390/zarch-omitted-base-index-err.s: Test cases
	for omitted base and/or index register.
	* testsuite/gas/s390/zarch-omitted-base-index-err.l: Likewise.

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