S/390: Make "b" constraint match literal pool references

S/390: Make "b" constraint match literal pool references

Improves the code generation by getting rid of redundant LAs, as seen
in the following example:

	-	la	%r1,0(%r13)
	-	lg	%r4,0(%r1)
	+	lg	%r4,0(%r13)

Also allows to proceed with the merge of movdi_64 and movdi_larl.
Currently LRA decides to spill literal pool references back to the
literal pool, because it preliminarily chooses alternatives with
CT_MEMORY constraints without calling
satisfies_memory_constraint_p (). Later on it notices that the
constraint is wrong and fixes it by spilling.  The constraint in this
case is "b", and the operand is a literal pool reference.  There is
no reason to reject them.  The current behavior was introduced,
apparently unintentionally, by
https://gcc.gnu.org/ml/gcc-patches/2010-09/msg00812.html

The patch affects a little bit more than mentioned in the subject,
because it changes s390_loadrelative_operand_p (), which is called not
only for checking the "b" constraint.  However, the only caller for
which it should really not accept literal pool references is
s390_check_qrst_address (), so it was changed to explicitly do so.

gcc/ChangeLog:

2018-10-22  Ilya Leoshkevich  <iii@linux.ibm.com>

	* config/s390/s390.c (s390_loadrelative_operand_p): Accept
	literal pool references.
	(s390_check_qrst_address): Adapt to the new behavior of
	s390_loadrelative_operand_p ().

gcc/testsuite/ChangeLog:

2018-10-22  Ilya Leoshkevich  <iii@linux.ibm.com>

	* gcc.target/s390/litpool-int.c: New test.

From-SVN: r265371