Certain lengths used in an MVC instruction might disable operand
forwarding.  Split MVCs into up to 2 forwardable ones if possible.

gcc/ChangeLog:

2017-12-01  Andreas Krebbel  <krebbel@linux.vnet.ibm.com>

	* config/s390/predicates.md (plus16_Q_operand): New predicate.
	* config/s390/s390.md: Disable MVC merging peephole if it would
	disable operand forwarding.
	(new peephole2): Split MVCs if it would turn them into up to 2
	forwardable MVCs.

From-SVN: r255319